The Entity Framework 4.0 and ASP.NET Web Forms: Getting Started

Tom Dykstra

Summary: In this book, you’ll learn the basics of using Entity Framework Database First to display and edit data in an ASP.NET Web Forms application.

Category: Step-By-Step
Applies to: ASP.NET 4.0, ASP.NET Web Forms, Entity Framework 4.0, Visual Studio 2010
Source: ASP.NET site (link to source content)
E-book publication date: June 2012

Copyright © 2012 by Microsoft Corporation

All rights reserved. No part of the contents of this book may be reproduced or transmitted in any form or by any means without the written permission of the publisher.

Microsoft and the trademarks listed at http://www.microsoft.com/about/legal/en/us/IntellectualProperty/Trademarks/EN-US.aspx are trademarks of the Microsoft group of companies. All other marks are property of their respective owners.

The example companies, organizations, products, domain names, email addresses, logos, people, places, and events depicted herein are fictitious. No association with any real company, organization, product, domain name, email address, logo, person, place, or event is intended or should be inferred.

This book expresses the author’s views and opinions. The information contained in this book is provided without any express, statutory, or implied warranties. Neither the authors, Microsoft Corporation, nor its resellers, or distributors will be held liable for any damages caused or alleged to be caused either directly or indirectly by this book.

Contents

Introduction

Creating the Web Application

Creating the Database

Creating the Entity Framework Data Model

Exploring the Entity Framework Data Model

The EntityDataSource Control

Adding and Configuring the EntityDataSource Control

Configuring Database Rules to Allow Deletion

Using a GridView Control to Read and Update Entities

Revising EntityDataSource Control Markup to Improve Performance

Displaying Data from a Navigation Property

Using a DetailsView Control to Insert Entities

Displaying Data in a Drop-Down List

Filtering, Ordering, and Grouping Data

Using the EntityDataSource “Where” Property to Filter Data

Using the EntityDataSource “OrderBy” Property to Order Data

Using a Control Parameter to Set the “Where” Property

Using the EntityDataSource “GroupBy” Property to Group Data

Using the QueryExtender Control for Filtering and Ordering

Using the “Like” Operator to Filter Data

Working with Related Data

Displaying and Updating Related Entities in a GridView Control

Displaying Related Entities in a Separate Control

Using the EntityDataSource “Selected” Event to Display Related Data

Working with Related Data, Continued

Adding an Entity with a Relationship to an Existing Entity

Working with Many-to-Many Relationships

Implementing Table-per-Hierarchy Inheritance

Table-per-Hierarchy versus Table-per-Type Inheritance

Adding Instructor and Student Entities

Mapping Instructor and Student Entities to the Person Table

Using the Instructor and Student Entities

Using Stored Procedures

Creating Stored Procedures in the Database

Adding the Stored Procedures to the Data Model

Mapping the Stored Procedures

Using Insert, Update, and Delete Stored Procedures

Using Select Stored Procedures

Using Dynamic Data Functionality to Format and Validate Data

Using DynamicField and DynamicControl Controls

Adding Metadata to the Data Model

The ObjectDataSource Control

Business Logic and Repository classes

Updating the Database and the Data Model

Adding a Relationship to the Database

Adding a View to the Database

Updating the Data Model

Using a Repository Class and an ObjectDataSource Control

Adding Insert and Delete Functionality

The Attach Method

The SaveChanges Method

Retrieving Instructor Names to Select When Inserting

Creating a Page for Inserting Departments

Adding Update Functionality

Adding a Business Logic Layer and Unit Tests

Creating a Repository Interface

Creating a Business-Logic Class

Creating a Unit-Test Project and Repository Implementation

Creating Unit Tests

Adding Business Logic to Make a Test Pass

Handling ObjectDataSource Exceptions

Sorting and Filtering

Adding the Ability to Sort GridView Columns

Adding a Search Box

Adding a Details Column for Each Grid Row

Handling Concurrency

Concurrency Conflicts

Pessimistic Concurrency (Locking)

Optimistic Concurrency

Detecting Concurrency Conflicts

Handling Optimistic Concurrency Without a Tracking Property

Enabling Concurrency Tracking in the Data Model

Handling Concurrency Exceptions in the DAL

Handling Concurrency Exceptions in the Presentation Layer

Testing Optimistic Concurrency in the Departments Page

Handling Optimistic Concurrency Using a Tracking Property

Adding OfficeAssignment Stored Procedures to the Data Model

Adding OfficeAssignment Methods to the DAL

Adding OfficeAssignment Methods to the BLL

Creating an OfficeAssignments Web Page

Testing Optimistic Concurrency in the OfficeAssignments Page

Handling Concurrency with the EntityDataSource Control

Maximizing Performance

Efficiently Loading Related Data

Managing View State

Using The NoTracking Merge Option

Pre-Compiling LINQ Queries

Examining Queries Sent to the Database

Pre-Generating Views

What’s New in the Entity Framework 4

Foreign-Key Associations

Executing User-Defined SQL Commands

Model-First Development

POCO Support

Code-First Development

More Information

Introduction

The application you’ll be building in these tutorials is a simple university website.

Users can view and update student, course, and instructor information. A few of the screens you’ll create are shown below.

Creating the Web Application

To start the tutorial, open Visual Studio and then create a new ASP.NET Web Application Project using the ASP.NET Web Application template:

This template creates a web application project that already includes a style sheet and master pages:

Open the Site.Master file and change “My ASP.NET Application” to “Contoso University”.

<h1>
Contoso University
</h1>

Click here to view code as image

Find the Menu control named NavigationMenu and replace it with the following markup, which adds menu items for the pages you’ll be creating.

<asp:Menu ID="NavigationMenu" runat="server" CssClass="menu"
EnableViewState="false"
IncludeStyleBlock="false" Orientation="Horizontal">
<Items>
<asp:MenuItem NavigateUrl="/Default.aspx" Text="Home" />

<asp:MenuItem NavigateUrl="/About.aspx" Text="About" />
<asp:MenuItem NavigateUrl="/Students.aspx" Text="Students">
<asp:MenuItem NavigateUrl="/StudentsAdd.aspx" Text="Add
Students" />
</asp:MenuItem>
<asp:MenuItem NavigateUrl="/Courses.aspx" Text="Courses">
<asp:MenuItem NavigateUrl="/CoursesAdd.aspx" Text="Add
Courses" />
</asp:MenuItem>
<asp:MenuItem NavigateUrl="/Instructors.aspx"
Text="Instructors">
<asp:MenuItem NavigateUrl="/InstructorsCourses.aspx"
Text="Course Assignments" />
<asp:MenuItem NavigateUrl="/OfficeAssignments.aspx"
Text="Office Assignments" />
</asp:MenuItem>
<asp:MenuItem NavigateUrl="/Departments.aspx"
Text="Departments">
<asp:MenuItem NavigateUrl="/DepartmentsAdd.aspx"
Text="Add Departments" />
</asp:MenuItem>
</Items>
</asp:Menu>

Click here to view code as image

Open the Default.aspx page and change the Content control named BodyContent to this:

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<h2>
Welcome to Contoso University!
</h2>
</asp:Content>

Click here to view code as image

You now have a simple home page with links to the various pages that you’ll be creating:

Creating the Database

For these tutorials, you’ll use the Entity Framework data model designer to automatically create the data model based on an existing database (often called the database-first approach). An alternative that’s not covered in this tutorial series is to create the data model manually and then have the designer generate scripts that create the database (the model-first approach).

For the database-first method used in this tutorial, the next step is to add a database to the site. The easiest way is to first download the project that goes with this tutorial. Then right-click the App_Data folder, select Add Existing Item, and select the School.mdf database file from the downloaded project.

An alternative is to follow the instructions at Creating the School Sample Database. Whether you download the database or create it, copy the School.mdf file from the following folder to your application’s App_Data folder:

%PROGRAMFILES%\Microsoft SQL Server\MSSQL10.SQLEXPRESS\MSSQL\DATA

(This location of the .mdf file assumes you’re using SQL Server 2008 Express.)

If you create the database from a script, perform the following steps to create a database diagram:

1. In Server Explorer, expand Data Connections, expand School.mdf, right-click Database Diagrams, and select Add New Diagram.

2. Select all of the tables and then click Add.

SQL Server creates a database diagram that shows tables, columns in the tables, and relationships between the tables. You can move the tables around to organize them however you like.

3. Save the diagram as “SchoolDiagram” and close it.

If you download the School.mdf file that goes with this tutorial, you can view the database diagram by double-clicking SchoolDiagram under Database Diagrams in Server Explorer.

The diagram looks something like this (the tables might be in different locations from what’s shown here):

Creating the Entity Framework Data Model

Now you can create an Entity Framework data model from this database. You could create the data model in the root folder of the application, but for this tutorial you’ll place it in a folder named DAL (for Data Access Layer).

In Solution Explorer, add a project folder named DAL (make sure it’s under the project, not under the solution).

Right-click the DAL folder and then select Add and New Item. Under Installed Templates, select Data, select the ADO.NET Entity Data Model template, name it SchoolModel.edmx, and then click Add.

This starts the Entity Data Model Wizard. In the first wizard step, the Generate from database option is selected by default. Click Next.

In the Choose Your Data Connection step, leave the default values and click Next. The School database is selected by default and the connection setting is saved in the Web.config file as SchoolEntities.

In the Choose Your Database Objects wizard step, select all of the tables except sysdiagrams (which was created for the diagram you generated earlier) and then click Finish.

After it’s finished creating the model, Visual Studio shows you a graphical representation of the Entity Framework objects (entities) that correspond to your database tables. (As with the database diagram, the location of individual elements might be different from what you see in this illustration. You can drag the elements around to match the illustration if you want.)

Exploring the Entity Framework Data Model

You can see that the entity diagram looks very similar to the database diagram, with a couple of differences. One difference is the addition of symbols at the end of each association that indicate the type of association (table relationships are called entity associations in the data model):

• A one-to-zero-or-one association is represented by “1” and “0..1”.

In this case, a Person entity may or may not be associated with an OfficeAssignment entity. An OfficeAssignment entity must be associated with a Person entity. In other words, an instructor may or may not be assigned to an office, and any office can be assigned to only one instructor.

• A one-to-many association is represented by “1” and “*”.

In this case, a Person entity may or may not have associated StudentGrade entities. A StudentGrade entity must be associated with one Person entity. StudentGrade entities actually represent enrolled courses in this database; if a student is enrolled in a course and there’s no grade yet, the Grade property is null. In other words, a student may not be enrolled in any courses yet, may be enrolled in one course, or may be enrolled in multiple courses. Each grade in an enrolled course applies to only one student.

• A many-to-many association is represented by “*” and “*”.

In this case, a Person entity may or may not have associated Course entities, and the reverse is also true: a Course entity may or may not have associated Person entities. In other words, an instructor may teach multiple courses, and a course may be taught by multiple instructors. (In this database, this relationship applies only to instructors; it does not link students to courses. Students are linked to courses by the StudentGrades table.)

Another difference between the database diagram and the data model is the additional Navigation Properties section for each entity. A navigation property of an entity references related entities. For example, the Courses property in a Person entity contains a collection of all the Course entities that are related to that Person entity.

Yet another difference between the database and data model is the absence of the CourseInstructor association table that’s used in the database to link the Person and Course tables in a many-to-many relationship. The navigation properties enable you to get related Course entities from the Person entity and related Person entities from the Course entity, so there’s no need to represent the association table in the data model.

For purposes of this tutorial, suppose the FirstName column of the Person table actually contains both a person’s first name and middle name. You want to change the name of the field to reflect this, but the database administrator (DBA) might not want to change the database. You can change the name of the FirstName property in the data model, while leaving its database equivalent unchanged.

In the designer, right-click FirstName in the Person entity, and then select Rename.

Type in the new name “FirstMidName”. This changes the way you refer to the column in code without changing the database.

The model browser provides another way to view the database structure, the data model structure, and the mapping between them. To see it, right-click a blank area in the entity designer and then click Model Browser.

The Model Browser pane displays a tree view. (The Model Browser pane might be docked with the Solution Explorer pane.) The SchoolModel node represents the data model structure, and the SchoolModel.Store node represents the database structure.

Expand SchoolModel.Store to see the tables, expand Tables / Views to see tables, and then expand Course to see the columns within a table.

Expand SchoolModel, expand Entity Types, and then expand the Course node to see the entities and the properties within the entities.

In either the designer or the Model Browser pane you can see how the Entity Framework relates the objects of the two models. Right-click the Person entity and select Table Mapping.

This opens the Mapping Details window. Notice that this window lets you see that the database column FirstName is mapped to FirstMidName, which is what you renamed it to in the data model.

The Entity Framework uses XML to store information about the database, the data model, and the mappings between them. The SchoolModel.edmx file is actually an XML file that contains this information. The designer renders the information in a graphical format, but you can also view the file as XML by right-clicking the .edmx file in Solution Explorer, clicking Open With, and selecting XML (Text) Editor. (The data model designer and an XML editor are just two different ways of opening and working with the same file, so you cannot have the designer open and open the file in an XML editor at the same time.)

You’ve now created a website, a database, and a data model. In the next walkthrough you’ll begin working with data using the data model and the ASP.NET EntityDataSource control.

The EntityDataSource Control

In the previous tutorial you created a web site, a database, and a data model. In this tutorial you work with the EntityDataSource control that ASP.NET provides in order to make it easy to work with an Entity Framework data model. You’ll create a GridView control for displaying and editing student data, a DetailsView control for adding new students, and a DropDownList control for selecting a department (which you’ll use later for displaying associated courses).

Note that in this application you won’t be adding input validation to pages that update the database, and some of the error handling will not be as robust as would be required in a production application. That keeps the tutorial focused on the Entity Framework and keeps it from getting too long. For details about how to add these features to your application, see Validating User Input in ASP.NET Web Pages and Error Handling in ASP.NET Pages and Applications.

Adding and Configuring the EntityDataSource Control

You’ll begin by configuring an EntityDataSource control to read Person entities from the People entity set.

Make sure you have Visual Studio open and that you’re working with the project you created in part 1. If you haven’t built the project since you created the data model or since the last change you made to it, build the project now. Changes to the data model are not made available to the designer until the project is built.

Create a new web page using the Web Form using Master Page template, and name it Students.aspx.

Specify Site.Master as the master page. All of the pages you create for these tutorials will use this master page.

In Source view, add an h2 heading to the Content control named Content2, as shown in the following example:

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<h2>Student List</h2>
</asp:Content>

Click here to view code as image

From the Data tab of the Toolbox, drag an EntityDataSource control to the page, drop it below the heading, and change the ID to StudentsEntityDataSource:

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<h2>Student List</h2>
<asp:EntityDataSource ID="StudentsEntityDataSource" runat="server">
</asp:EntityDataSource>
</asp:Content>

Click here to view code as image

Switch to Design view, click the data source control’s smart tag, and then click Configure Data Source to launch the Configure Data Source wizard.

In the Configure ObjectContext wizard step, select SchoolEntities as the value for Named Connection, and select SchoolEntities as the DefaultContainerName value. Then click Next.

Note: If you get the following dialog box at this point, you have to build the project before proceeding.

In the Configure Data Selection step, select People as the value for EntitySetName. Under Select, make sure the Select All check box is selected. Then select the options to enable update and delete. When you’re done, click Finish.

Configuring Database Rules to Allow Deletion

You’ll be creating a page that lets users delete students from the Person table, which has three relationships with other tables (Course, StudentGrade, and OfficeAssignment). By default, the database will prevent you from deleting a row in Person if there are related rows in one of the other tables. You can manually delete the related rows first, or you can configure the database to delete them automatically when you delete a Person row. For student records in this tutorial, you’ll configure the database to delete the related data automatically. Because students can have related rows only in the StudentGrade table, you need to configure only one of the three relationships.

If you’re using the School.mdf file that you downloaded from the project that goes with this tutorial, you can skip this section because these configuration changes have already been done. If you created the database by running a script, configure the database by performing the following procedures.

In Server Explorer, open the database diagram that you created in part 1. Right-click the relationship between Person and StudentGrade (the line between tables), and then select Properties.

In the Properties window, expand INSERT and UPDATE Specification and set the DeleteRule property to Cascade.

Save and close the diagram. If you’re asked whether you want to update the database, click Yes.

To make sure that the model keeps entities that are in memory in sync with what the database is doing, you must set corresponding rules in the data model. Open SchoolModel.edmx, right-click the association line between Person and StudentGrade, and then select Properties.

In the Properties window, set End1 OnDelete to Cascade.

Save and close the SchoolModel.edmx file, and then rebuild the project.

In general, when the database changes, you have several choices for how to sync up the model:

• For certain kinds of changes (such as adding or refreshing tables, views, or stored procedures), right-click in the designer and select Update Model from Database to have the designer make the changes automatically.

• Regenerate the data model.

• Make manual updates like this one.

In this case, you could have regenerated the model or refreshed the tables affected by the relationship change, but then you’d have to make the field-name change again (from FirstName to FirstMidName).

Using a GridView Control to Read and Update Entities

In this section you’ll use a GridView control to display, update, or delete students.

Open or switch to Students.aspx and switch to Design view. From the Data tab of the Toolbox, drag a GridView control to the right of the EntityDataSource control, name it StudentsGridView, click the smart tag, and then select StudentsEntityDataSource as the data source.

Click Refresh Schema (click Yes if you’re prompted to confirm), then click Enable Paging, Enable Sorting, Enable Editing, and Enable Deleting.

Click Edit Columns.

In the Selected fields box, delete PersonID, LastName, and HireDate. You typically don’t display a record key to users, hire date is not relevant to students, and you’ll put both parts of the name in one field, so you only need one of the name fields.)

Select the FirstMidName field and then click Convert this field into a TemplateField.

Do the same for EnrollmentDate.

Click OK and then switch to Source view. The remaining changes will be easier to do directly in markup. The GridView control markup now looks like the following example.

<asp:GridView ID="StudentsGridView" runat="server" AllowPaging="True"
AllowSorting="True" AutoGenerateColumns="False" DataKeyNames="PersonID"
DataSourceID="StudentsEntityDataSource">
<Columns>
<asp:CommandField ShowDeleteButton="True" ShowEditButton="True" />
<asp:TemplateField HeaderText="FirstMidName"
SortExpression="FirstMidName">
<EditItemTemplate>
<asp:TextBox ID="TextBox1" runat="server" Text='<%#
Bind("FirstMidName") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%#
Bind("FirstMidName") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="EnrollmentDate"
SortExpression="EnrollmentDate">
<EditItemTemplate>
<asp:TextBox ID="TextBox2" runat="server" Text='<%#
Bind("EnrollmentDate") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="Label2" runat="server" Text='<%#
Bind("EnrollmentDate") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

Click here to view code as image

The first column after the command field is a template field that currently displays the first name. Change the markup for this template field to look like the following example:

<asp:TemplateField HeaderText="Name" SortExpression="LastName">
<EditItemTemplate>
<asp:TextBox ID="LastNameTextBox" runat="server" Text='<%#
Bind("LastName") %>'></asp:TextBox>
<asp:TextBox ID="FirstNameTextBox" runat="server" Text='<%#
Bind("FirstMidName") %>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="LastNameLabel" runat="server" Text='<%#
Eval("LastName") %>'></asp:Label>,
<asp:Label ID="FirstNameLabel" runat="server" Text='<%#
Eval("FirstMidName")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>

Click here to view code as image

In display mode, two Label controls display the first and last name. In edit mode, two text boxes are provided so you can change the first and last name. As with the Label controls in display mode, you use Bind and Eval expressions exactly as you would with ASP.NET data source controls that connect directly to databases. The only difference is that you’re specifying entity properties instead of database columns.

The last column is a template field that displays the enrollment date. Change the markup for this field to look like the following example:

<asp:TemplateField HeaderText="Enrollment Date"
SortExpression="EnrollmentDate">
<EditItemTemplate>
<asp:TextBox ID="EnrollmentDateTextBox" runat="server" Text='<%#
Bind("EnrollmentDate", "{0:d}")%>'></asp:TextBox>
</EditItemTemplate>
<ItemTemplate>
<asp:Label ID="EnrollmentDateLabel" runat="server" Text='<%#
Eval("EnrollmentDate", "{0:d}") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>

Click here to view code as image

In both display and edit mode, the format string “{0,d}” causes the date to be displayed in the “short date” format. (Your computer might be configured to display this format differently from the screen images shown in this tutorial.)

Notice that in each of these template fields, the designer used a Bind expression by default, but you’ve changed that to an Eval expression in the ItemTemplate elements. The Bind expression makes the data available in GridView control properties in case you need to access the data in code. In this page you don’t need to access this data in code, so you can use Eval, which is more efficient. For more information, see Getting your data out of the data controls.

Revising EntityDataSource Control Markup to Improve Performance

In the markup for the EntityDataSource control, remove the ConnectionString and DefaultContainerName attributes and replace them with a ContextTypeName="ContosoUniversity.DAL.SchoolEntities" attribute. This is a change you should make every time you create an EntityDataSource control, unless you need to use a connection that is different from the one that’s hard-coded in the object context class. Using the ContextTypeName attribute provides the following benefits:

• Better performance. When the EntityDataSource control initializes the data model using the ConnectionString and DefaultContainerName attributes, it performs additional work to load metadata on every request. This isn’t necessary if you specify the ContextTypeName attribute.

• Lazy loading is turned on by default in generated object context classes (such as SchoolEntities in this tutorial) in Entity Framework 4.0. This means that navigation properties are loaded with related data automatically right when you need it. Lazy loading is explained in more detail later in this tutorial.

• Any customizations that you’ve applied to the object context class (in this case, the SchoolEntities class) will be available to controls that use the EntityDataSource control. Customizing the object context class is an advanced topic that is not covered in this tutorial series. For more information, see Extending Entity Framework Generated Types.

The markup will now resemble the following example (the order of the properties might be different):

<asp:EntityDataSource ID="StudentsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People"
EnableDelete="True" EnableUpdate="True">
</asp:EntityDataSource>

Click here to view code as image

The EnableFlattening attribute refers to a feature that was needed in earlier versions of the Entity Framework because foreign key columns were not exposed as entity properties. The current version makes it possible to use foreign key associations, which means foreign key properties are exposed for all but many-to-many associations. If your entities have foreign key properties and no complex types, you can leave this attribute set to False. Don’t remove the attribute from the markup, because the default value is True. For more information, see Flattening Objects (EntityDataSource).

Run the page and you see a list of students and employees (you’ll filter for just students in the next tutorial). The first name and last name are displayed together.

To sort the display, click a column name.

Click Edit in any row. Text boxes are displayed where you can change the first and last name.

The Delete button also works. Click delete for a row that has an enrollment date and the row disappears. (Rows without an enrollment date represent instructors and you may get a referential integrity error. In the next tutorial you’ll filter this list to include just students.)

Displaying Data from a Navigation Property

Now suppose you want to know how many courses each student is enrolled in. The Entity Framework provides that information in the StudentGrades navigation property of the Person entity. Because the database design does not allow a student to be enrolled in a course without having a grade assigned, for this tutorial you can assume that having a row in the StudentGrade table row that is associated with a course is the same as being enrolled in the course. (The Courses navigation property is only for instructors.)

When you use the ContextTypeName attribute of the EntityDataSource control, the Entity Framework automatically retrieves information for a navigation property when you access that property. This is called lazy loading. However, this can be inefficient, because it results in a separate call to the database each time additional information is needed. If you need data from the navigation property for every entity returned by the EntityDataSource control, it’s more efficient to retrieve the related data along with the entity itself in a single call to the database. This is called eager loading, and you specify eager loading for a navigation property by setting the Include property of the EntityDataSource control.

In Students.aspx, you want to show the number of courses for every student, so eager loading is the best choice. If you were displaying all students but showing the number of courses only for a few of them (which would require writing some code in addition to the markup), lazy loading might be a better choice.

Open or switch to Students.aspx, switch to Design view, select StudentsEntityDataSource, and in the Properties window set the Include property to StudentGrades. (If you wanted to get multiple navigation properties, you could specify their names separated by commas — for example, StudentGrades, Courses.)

Switch to Source view. In the StudentsGridView control, after the last asp:TemplateField element, add the following new template field:

<asp:TemplateField HeaderText="Number of Courses">
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%#
Eval("StudentGrades.Count")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>

Click here to view code as image

In the Eval expression, you can reference the navigation property StudentGrades. Because this property contains a collection, it has a Count property that you can use to display the number of courses in which the student is enrolled. In a later tutorial you’ll see how to display data from navigation properties that contain single entities instead of collections. (Note that you cannot use BoundField elements to display data from navigation properties.)

Run the page and you now see how many courses each student is enrolled in.

Using a DetailsView Control to Insert Entities

The next step is to create a page that has a DetailsView control that will let you add new students. Close the browser and then create a new web page using the Site.Master master page. Name the page StudentsAdd.aspx, and then switch to Source view.

Add the following markup to replace the existing markup for the Content control named Content2:

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<h2>Add New Students</h2>
<asp:EntityDataSource ID="StudentsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EnableInsert="True" EntitySetName="People">
</asp:EntityDataSource>
<asp:DetailsView ID="StudentsDetailsView" runat="server"
DataSourceID="StudentsEntityDataSource" AutoGenerateRows="False"
DefaultMode="Insert">
<Fields>
<asp:BoundField DataField="FirstMidName" HeaderText="First Name"
SortExpression="FirstMidName" />
<asp:BoundField DataField="LastName" HeaderText="Last Name"
SortExpression="LastName" />
<asp:BoundField DataField="EnrollmentDate" HeaderText="Enrollment Date"
SortExpression="EnrollmentDate" />
<asp:CommandField ShowInsertButton="True" />
</Fields>
</asp:DetailsView>
</asp:Content>

Click here to view code as image

This markup creates an EntityDataSource control that is similar to the one you created in Students.aspx, except it enables insertion. As with the GridView control, the bound fields of the DetailsView control are coded exactly as they would be for a data control that connects directly to a database, except that they reference entity properties. In this case, the DetailsView control is used only for inserting rows, so you have set the default mode to Insert.

Run the page and add a new student.

Nothing will happen after you insert a new student, but if you now run Students.aspx, you’ll see the new student information.

Displaying Data in a Drop-Down List

In the following steps you’ll databind a DropDownList control to an entity set using an EntityDataSource control. In this part of the tutorial, you won’t do much with this list. In subsequent parts, though, you’ll use the list to let users select a department to display courses associated with the department.

Create a new web page named Courses.aspx. In Source view, add a heading to the Content control that’s named Content2:

<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
<h2>Courses by Department</h2>
</asp:Content>

Click here to view code as image

In Design view, add an EntityDataSource control to the page as you did before, except this time name it DepartmentsEntityDataSource. Select Departments as the EntitySetName value, and select only the DepartmentID and Name properties.

From the Standard tab of the Toolbox, drag a DropDownList control to the page, name it DepartmentsDropDownList, click the smart tag, and select Choose Data Source to start the DataSource Configuration Wizard.

In the Choose a Data Source step, select DepartmentsEntityDataSource as the data source, click Refresh Schema, and then select Name as the data field to display and DepartmentID as the value data field. Click OK.

The method you use to databind the control using the Entity Framework is the same as with other ASP.NET data source controls except you’re specifying entities and entity properties.

Switch to Source view and add “Select a department:” immediately before the DropDownList control.

Select a department:
<asp:DropDownList ID="DropDownList1" runat="server"
DataSourceID="EntityDataSource1" DataTextField="Name"
DataValueField="DepartmentID">
</asp: DropDownList>

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As a reminder, change the markup for the EntityDataSource control at this point by replacing the ConnectionString and DefaultContainerName attributes with a ContextTypeName="ContosoUniversity.DAL.SchoolEntities" attribute. It’s often best to wait until after you’ve created the data-bound control that is linked to the data source control before you change the EntityDataSource control markup, because after you make the change, the designer will not provide you with a Refresh Schema option in the data-bound control.

Run the page and you can select a department from the drop-down list.

This completes the introduction to using the EntityDataSource control. Working with this control is generally no different from working with other ASP.NET data source controls, except that you reference entities and properties instead of tables and columns. The only exception is when you want to access navigation properties. In the next tutorial you’ll see that the syntax you use with EntityDataSource control might also differ from other data source controls when you filter, group, and order data.

Filtering, Ordering, and Grouping Data

In the previous tutorial you used the EntityDataSource control to display and edit data. In this tutorial you’ll filter, order, and group data. When you do this by setting properties of the EntityDataSource control, the syntax is different from other data source controls. As you’ll see, however, you can use the QueryExtender control to minimize these differences.

You’ll change the Students.aspx page to filter for students, sort by name, and search on name. You’ll also change the Courses.aspx page to display courses for the selected department and search for courses by name. Finally, you’ll add student statistics to the About.aspx page.

Using the EntityDataSource “Where” Property to Filter Data

Open the Students.aspx page that you created in the previous tutorial. As currently configured, the GridView control in the page displays all the names from the People entity set. However, you want to show only students, which you can find by selecting Person entities that have non-null enrollment dates.

Switch to Design view and select the EntityDataSource control. In the Properties window, set the Where property to it.EnrollmentDate is not null.

The syntax you use in the Where property of the EntityDataSource control is Entity SQL. Entity SQL is similar to Transact-SQL, but it’s customized for use with entities rather than database objects. In the expression it.EnrollmentDate is not null, the word it represents a reference to the entity returned by the query. Therefore, it.EnrollmentDate refers to the EnrollmentDate property of the Person entity that the EntityDataSource control returns.

Run the page. The students list now contains only students. (There are no rows displayed where there’s no enrollment date.)

Using the EntityDataSource “OrderBy” Property to Order Data

You also want this list to be in name order when it’s first displayed. With the Students.aspx page still open in Design view, and with the EntityDataSource control still selected, in the Properties window set the OrderBy property to it.LastName.

Run the page. The students list is now in order by last name.

Using a Control Parameter to Set the “Where” Property

As with other data source controls, you can pass parameter values to the Where property. On the Courses.aspx page that you created in part 2 of the tutorial, you can use this method to display courses that are associated with the department that a user selects from the drop-down list.

Open Courses.aspx and switch to Design view. Add a second EntityDataSource control to the page, and name it CoursesEntityDataSource. Connect it to the SchoolEntities model, and select Courses as the EntitySetName value.

In the Properties window, click the ellipsis in the Where property box. (Make sure the CoursesEntityDataSource control is still selected before using the Properties window.)

The Expression Editor dialog box is displayed. In this dialog box, select Automatically generate the Where expression based on the provided parameters, and then click Add Parameter. Name the parameter DepartmentID, select Control as the Parameter source value, and select DepartmentsDropDownList as the ControlID value.

Click Show advanced properties, and in the Properties window of the Expression Editor dialog box, change the Type property to Int32.

When you’re done, click OK.

Below the drop-down list, add a GridView control to the page and name it CoursesGridView. Connect it to the CoursesEntityDataSource data source control, click Refresh Schema, click Edit Columns, and remove the DepartmentID column. The GridView control markup resembles the following example.

<asp:GridView ID="CoursesGridView" runat="server" AutoGenerateColumns="False"
DataKeyNames="CourseID" DataSourceID="CoursesEntityDataSource">
<Columns>
<asp:BoundField DataField="CourseID" HeaderText="ID" ReadOnly="True"
SortExpression="CourseID" />
<asp:BoundField DataField="Title" HeaderText="Title"
SortExpression="Title" />
<asp:BoundField DataField="Credits" HeaderText="Credits"
SortExpression="Credits" />
</Columns>
</asp:GridView>

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When the user changes the selected department in the drop-down list, you want the list of associated courses to change automatically. To make this happen, select the drop-down list, and in the Properties window set the AutoPostBack property to True.

Now that you’re finished using the designer, switch to Source view and replace the ConnectionString and DefaultContainer name properties of the CoursesEntityDataSource control with the ContextTypeName="ContosoUniversity.DAL.SchoolEntities" attribute. When you’re done, the markup for the control will look like the following example.

<asp:EntityDataSource ID="CoursesEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="false"
EntitySetName="Courses"
AutoGenerateWhereClause="true" Where="">
<WhereParameters>
<asp:ControlParameter ControlID="DepartmentsDropDownList" Type="Int32"
Name="DepartmentID" PropertyName="SelectedValue" />
</WhereParameters>
</asp:EntityDataSource>

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Run the page and use the drop-down list to select different departments. Only courses that are offered by the selected department are displayed in the GridView control.

Using the EntityDataSource “GroupBy” Property to Group Data

Suppose Contoso University wants to put some student-body statistics on its About page. Specifically, it wants to show a breakdown of numbers of students by the date they enrolled.

Open About.aspx, and in Source view, replace the existing contents of the BodyContent control with “Student Body Statistics” between h2 tags:

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<h2>Student Body Statistics</h2>
</asp:Content>

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After the heading, add an EntityDataSource control and name it StudentStatisticsEntityDataSource. Connect it to SchoolEntities, select the People entity set, and leave the Select box in the wizard unchanged. Set the following properties in the Properties window:

• To filter for students only, set the Where property to it.EnrollmentDate is not null.

• To group the results by the enrollment date, set the GroupBy property to it.EnrollmentDate.

• To select the enrollment date and the number of students, set the Select property to it.EnrollmentDate, Count(it.EnrollmentDate) AS NumberOfStudents.

• To order the results by the enrollment date, set the OrderBy property to it.EnrollmentDate.

In Source view, replace the ConnectionString and DefaultContainer name properties with a ContextTypeName property. The EntityDataSource control markup now resembles the following example.

<asp:EntityDataSource ID="StudentStatisticsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People"
Select="it.EnrollmentDate, Count(it.EnrollmentDate) AS NumberOfStudents"
OrderBy="it.EnrollmentDate" GroupBy="it.EnrollmentDate"
Where="it.EnrollmentDate is not null" >
</asp:EntityDataSource>

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The syntax of the Select, GroupBy, and Where properties resembles Transact-SQL except for the it keyword that specifies the current entity.

Add the following markup to create a GridView control to display the data.

<asp:GridView ID="StudentStatisticsGridView" runat="server"
AutoGenerateColumns="False"
DataSourceID="StudentStatisticsEntityDataSource">
<Columns>
<asp:BoundField DataField="EnrollmentDate" DataFormatString="{0:d}"
HeaderText="Date of Enrollment"
ReadOnly="True" SortExpression="EnrollmentDate" />
<asp:BoundField DataField="NumberOfStudents" HeaderText="Students"
ReadOnly="True" SortExpression="NumberOfStudents" />
</Columns>
</asp:GridView>

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Run the page to see a list showing the number of students by enrollment date.

Using the QueryExtender Control for Filtering and Ordering

The QueryExtender control provides a way to specify filtering and sorting in markup. The syntax is independent of the database management system (DBMS) you’re using. It’s also generally independent of the Entity Framework, with the exception that syntax you use for navigation properties is unique to the Entity Framework.

In this part of the tutorial you’ll use a QueryExtender control to filter and order data, and one of the order-by fields will be a navigation property.

(If you prefer to use code instead of markup to extend the queries that are automatically generated by the EntityDataSource control, you can do that by handling the QueryCreated event. This is how the QueryExtender control extends EntityDataSource control queries also.)

Open the Courses.aspx page, and below the markup you added previously, insert the following markup to create a heading, a text box for entering search strings, a search button, and an EntityDataSource control that’s bound to the Courses entity set.

<h2>Courses by Name</h2>
Enter a course name
<asp:TextBox ID="SearchTextBox" runat="server"></asp:TextBox>
<asp:Button ID="SearchButton" runat="server" Text="Search" />
<br /><br />
<asp:EntityDataSource ID="SearchEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="Courses"
Include="Department" >
</asp:EntityDataSource>

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Notice that the EntityDataSource control’s Include property is set to Department. In the database, the Course table does not contain the department name; it contains a DepartmentID foreign key column. If you were querying the database directly, to get the department name along with course data, you would have to join the Course and Department tables. By setting the Include property to Department, you specify that the Entity Framework should do the work of getting the related Department entity when it gets a Course entity. The Department entity is then stored in the Department navigation property of the Course entity. (By default, the SchoolEntities class that was generated by the data model designer retrieves related data when it’s needed, and you’ve bound the data source control to that class, so setting the Include property is not necessary. However, setting it improves performance of the page, because otherwise the Entity Framework would make separate calls to the database to retrieve data for the Course entities and for the related Department entities.)

After the EntityDataSource control you just created, insert the following markup to create a QueryExtender control that’s bound to that EntityDataSource control.

<asp:QueryExtender ID="SearchQueryExtender" runat="server"
TargetControlID="SearchEntityDataSource" >
<asp:SearchExpression SearchType="StartsWith" DataFields="Title">
<asp:ControlParameter ControlID="SearchTextBox" />
</asp:SearchExpression>
<asp:OrderByExpression DataField="Department.Name" Direction="Ascending">
<asp:ThenBy DataField="Title" Direction="Ascending" />
</asp:OrderByExpression>
</asp:QueryExtender>

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The SearchExpression element specifies that you want to select courses whose titles match the value entered in the text box. Only as many characters as are entered in the text box will be compared, because the SearchType property specifies StartsWith.

The OrderByExpression element specifies that the result set will be ordered by course title within department name. Notice how department name is specified: Department.Name. Because the association between the Course entity and the Department entity is one-to-one, the Department navigation property contains a Department entity. (If this were a one-to-many relationship, the property would contain a collection.) To get the department name, you must specify the Name property of the Department entity.

Finally, add a GridView control to display the list of courses:

<asp:GridView ID="SearchGridView" runat="server" AutoGenerateColumns="False"
DataKeyNames="CourseID" DataSourceID="SearchEntityDataSource"
AllowPaging="true">
<Columns>
<asp:TemplateField HeaderText="Department">
<ItemTemplate>
<asp:Label ID="Label2" runat="server" Text='<%#
Eval("Department.Name")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:BoundField DataField="CourseID" HeaderText="ID"/>
<asp:BoundField DataField="Title" HeaderText="Title" />
<asp:BoundField DataField="Credits" HeaderText="Credits" />
</Columns>
</asp:GridView>

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The first column is a template field that displays the department name. The databinding expression specifies Department.Name, just as you saw in the QueryExtender control.

Run the page. The initial display shows a list of all courses in order by department and then by course title.

Enter an “m” and click Search to see all courses whose titles begin with “m” (the search is not case sensitive).

Using the “Like” Operator to Filter Data

You can achieve an effect similar to the QueryExtender control’s StartsWith, Contains, and EndsWith search types by using a Like operator in the EntityDataSource control’s Where property. In this part of the tutorial, you’ll see how to use the Like operator to search for a student by name.

Open Students.aspx in Source view. After the GridView control, add the following markup:

<h2>Find Students by Name</h2>
Enter any part of the name
<asp:TextBox ID="SearchTextBox" runat="server" AutoPostBack="true"></asp:TextBox>
<asp:Button ID="SearchButton" runat="server" Text="Search" />
<br />
<br />
<asp:EntityDataSource ID="SearchEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People"
Where="it.EnrollmentDate is not null and (it.FirstMidName Like '%' +
@StudentName + '%' or it.LastName Like '%' + @StudentName + '%')" >
<WhereParameters>
<asp:ControlParameter ControlID="SearchTextBox" Name="StudentName"
PropertyName="Text"
Type="String" DefaultValue="%"/>
</WhereParameters>
</asp:EntityDataSource>
<asp:GridView ID="SearchGridView" runat="server" AutoGenerateColumns="False"
DataKeyNames="PersonID"
DataSourceID="SearchEntityDataSource" AllowPaging="true">
<Columns>
<asp:TemplateField HeaderText="Name" SortExpression="LastName,
FirstMidName">
<ItemTemplate>
<asp:Label ID="LastNameFoundLabel" runat="server" Text='<%#
Eval("LastName")%>'></asp:Label>,
<asp:Label ID="FirstNameFoundLabel" runat="server" Text='<%#
Eval("FirstMidName") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Enrollment Date"
SortExpression="EnrollmentDate">
<ItemTemplate>
<asp:Label ID="EnrollmentDateFoundLabel" runat="server" Text='<%#
Eval("EnrollmentDate", "{0:d}")%>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

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This markup is similar to what you’ve seen earlier except for the Where property value. The second part of the Where expression defines a substring search (LIKE %FirstMidName% or LIKE %LastName%) that searches both the first and last names for whatever is entered in the text box.

Run the page. Initially you see all of the students because the default value for the StudentName parameter is “%”.

Enter the letter “g” in the text box and click Search. You see a list of students that have a “g” in either the first or last name.

You’ve now displayed, updated, filtered, ordered, and grouped data from individual tables. In the next tutorial you’ll begin to work with related data (master-detail scenarios).

Working with Related Data

In the previous tutorial you used the EntityDataSource control to filter, sort, and group data. In this tutorial you’ll display and update related data.

You’ll create the Instructors page that shows a list of instructors. When you select an instructor, you see a list of courses taught by that instructor. When you select a course, you see details for the course and a list of students enrolled in the course. You can edit the instructor name, hire date, and office assignment. The office assignment is a separate entity set that you access through a navigation property.

You can link master data to detail data in markup or in code. In this part of the tutorial, you’ll use both methods.

Displaying and Updating Related Entities in a GridView Control

Create a new web page named Instructors.aspx that uses the Site.Master master page, and add the following markup to the Content control named Content2:

<h2>Instructors</h2>
<div>
<asp:EntityDataSource ID="InstructorsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People"
Where="it.HireDate is not null" Include="OfficeAssignment"
EnableUpdate="True">
</asp:EntityDataSource>
</div>

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This markup creates an EntityDataSource control that selects instructors and enables updates. The div element configures markup to render on the left so that you can add a column on the right later.

Between the EntityDataSource markup and the closing </div> tag, add the following markup that creates a GridView control and a Label control that you’ll use for error messages:

<asp:GridView ID="InstructorsGridView" runat="server" AllowPaging="True"
AllowSorting="True"
AutoGenerateColumns="False" DataKeyNames="PersonID"
DataSourceID="InstructorsEntityDataSource"
OnSelectedIndexChanged="InstructorsGridView_SelectedIndexChanged"
SelectedRowStyle-BackColor="LightGray"
onrowupdating="InstructorsGridView_RowUpdating">
<Columns>
<asp:CommandField ShowSelectButton="True" ShowEditButton="True" />
<asp:TemplateField HeaderText="Name" SortExpression="LastName">
<ItemTemplate>
<asp:Label ID="InstructorLastNameLabel" runat="server"
Text='<%#Eval("LastName") %>'></asp:Label>,
<asp:Label ID="InstructorFirstNameLabel" runat="server"
Text='<%#Eval("FirstMidName") %>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="InstructorLastNameTextBox" runat="server"
Text='<%#Bind("FirstMidName") %>' Width="7em"></asp:TextBox>
<asp:TextBox ID="InstructorFirstNameTextBox" runat="server"
Text='<%#Bind("LastName") %>' Width="7em"></asp:TextBox>
</EditItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Hire Date" SortExpression="HireDate">
<ItemTemplate>
<asp:Label ID="InstructorHireDateLabel" runat="server"
Text='<%#Eval("HireDate", "{0:d}") %>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="InstructorHireDateTextBox" runat="server"
Text='<%#Bind("HireDate", "{0:d}") %>' Width="7em"></asp:TextBox>
</EditItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Office Assignment"
SortExpression="OfficeAssignment.Location">
<ItemTemplate>
<asp:Label ID="InstructorOfficeLabel" runat="server"
Text='<%#Eval("OfficeAssignment.Location") %>'></asp:Label>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox ID="InstructorOfficeTextBox" runat="server"
Text='<%#Eval("OfficeAssignment.Location") %>' Width="7em"
oninit="InstructorOfficeTextBox_Init"></asp:TextBox>
</EditItemTemplate>
</asp:TemplateField>
</Columns>
<SelectedRowStyle BackColor="LightGray"></SelectedRowStyle>
</asp:GridView>
<asp:Label ID="ErrorMessageLabel" runat="server" Text="" Visible="false"
ViewStateMode="Disabled"></asp:Label>

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This GridView control enables row selection, highlights the selected row with a light gray background color, and specifies handlers (which you’ll create later) for the SelectedIndexChanged and Updating events. It also specifies PersonID for the DataKeyNames property, so that the key value of the selected row can be passed to another control that you’ll add later.

The last column contains the instructor’s office assignment, which is stored in a navigation property of the Person entity because it comes from an associated entity. Notice that the EditItemTemplate element specifies Eval instead of Bind, because the GridView control cannot directly bind to navigation properties in order to update them. You’ll update the office assignment in code. To do that, you’ll need a reference to the TextBox control, and you’ll get and save that in the TextBox control’s Init event.

Following the GridView control is a Label control that’s used for error messages. The control’s Visible property is false, and view state is turned off, so that the label will appear only when code makes it visible in response to an error.

Open the Instructors.aspx.cs file and add the following using statement:

using ContosoUniversity.DAL;

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Add a private class field immediately after the partial-class name declaration to hold a reference to the office assignment text box.

private TextBox instructorOfficeTextBox;

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Add a stub for the SelectedIndexChanged event handler that you’ll add code for later. Also add a handler for the office assignment TextBox control’s Init event so that you can store a reference to the TextBox control. You’ll use this reference to get the value the user entered in order to update the entity associated with the navigation property.

protected void InstructorsGridView_SelectedIndexChanged(object sender,
EventArgs e)
{
}

protected void InstructorOfficeTextBox_Init(object sender, EventArgs e)
{
instructorOfficeTextBox=sender as TextBox;
}

Click here to view code as image

You’ll use the GridView control’s Updating event to update the Location property of the associated OfficeAssignment entity. Add the following handler for the Updating event:

protected void InstructorsGridView_RowUpdating(object sender,
GridViewUpdateEventArgs e)
{
using (var context= new SchoolEntities())
{
var instructorBeingUpdated=Convert. ToInt32(e. Keys[0]);
var officeAssignment = (from o in context. OfficeAssignments
where o. InstructorID ==
instructorBeingUpdated
select o). FirstOrDefault();
try
{
if ( String. IsNullOrWhiteSpace(instructorOfficeTextBox. Text) ==
false)
{
if (officeAssignment == null)
{

context. OfficeAssignments. AddObject( OfficeAssignment. CreateOfficeAssignment(instructo
rBeingUpdated, instructorOfficeTextBox. Text, null));
}
else
{
officeAssignment. Location = instructorOfficeTextBox. Text;
}
}
else
{
if (officeAssignment != null)
{
context. DeleteObject(officeAssignment);
}
}
context.SaveChanges();
}
catch ( Exception )
{
e. Cancel = true;
ErrorMessageLabel. Visible = true;
ErrorMessageLabel. Text="Update failed.";
//Add code to log the error.
}
}
}

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This code is run when the user clicks Update in a GridView row. The code uses LINQ to Entities to retrieve the OfficeAssignment entity that’s associated with the current Person entity, using the PersonID of the selected row from the event argument.

The code then takes one of the following actions depending on the value in the InstructorOfficeTextBox control:

• If the text box has a value and there’s no OfficeAssignment entity to update, it creates one.

• If the text box has a value and there’s an OfficeAssignment entity, it updates the Location property value.

• If the text box is empty and an OfficeAssignment entity exists, it deletes the entity.

After this, it saves the changes to the database. If an exception occurs, it displays an error message.

Run the page.

Click Edit and all fields change to text boxes.

Change any of these values, including Office Assignment. Click Update and you’ll see the changes reflected in the list.

Displaying Related Entities in a Separate Control

Each instructor can teach one or more courses, so you’ll add an EntityDataSource control and a GridView control to list the courses associated with whichever instructor is selected in the instructors GridView control. To create a heading and the EntityDataSource control for courses entities, add the following markup between the error message Label control and the closing </div> tag:

<h3>Courses Taught</h3>
<asp:EntityDataSource ID="CoursesEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="Courses"
Where="@PersonID IN (SELECT VALUE instructor.PersonID FROM it.People AS
instructor)">
<WhereParameters>
<asp:ControlParameter ControlID="InstructorsGridView" Type="Int32"
Name="PersonID" PropertyName="SelectedValue" />
</WhereParameters>
</asp:EntityDataSource>

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The Where parameter contains the value of the PersonID of the instructor whose row is selected in the InstructorsGridView control. The Where property contains a subselect command that gets all associated Person entities from a Course entity’s People navigation property and selects the Course entity only if one of the associated Person entities contains the selected PersonID value.

To create the GridView control., add the following markup immediately following the CoursesEntityDataSource control (before the closing </div> tag):

<asp:GridView ID="CoursesGridView" runat="server"
DataaSourceID="CoursesEntityDataSource"
AllowSorting="True" AutoGenerateColumns="False"
SelectedRowStyle-BackColor="LightGray"
DataKeyNames="CourseID">
<EmptyDataTemplate>
<p>No courses found.</p>
</EmptyDataTemplate>
<Columns>
<asp:CommandField ShowSelectButton="True" />
<asp:BoundField DataField="CourseID" HeaderText="ID" ReadOnly="True"
SortExpression="CourseID" />
<asp:BoundField DataField="Title" HeaderText="Title"
SortExpression="Title" />
<asp:TemplateField HeaderText="Department"
SortExpression="DepartmentID">
<ItemTemplate>
<asp:Label ID="GridViewDepartmentLabel" runat="server"
Text='<%#Eval("Department.Name") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

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Because no courses will be displayed if no instructor is selected, an EmptyDataTemplate element is included.

Run the page.

Select an instructor who has one or more courses assigned, and the course or courses appear in the list. (Note: although the database schema allows multiple courses, in the test data supplied with the database no instructor actually has more than one course. You can add courses to the database yourself using the Server Explorer window or the CoursesAdd.aspx page, which you’ll add in a later tutorial.)

The CoursesGridView control shows only a few course fields. To display all the details for a course, you’ll use a DetailsView control for the course that the user selects. In Instructors.aspx, add the following markup after the closing </div> tag (make sure you place this markup after the closing div tag, not before it):

<div>
<h3>Course Details</h3>
<asp:EntityDataSource ID="CourseDetailsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="Courses"
AutoGenerateWhereClause="False" Where="it.CourseID = @CourseID"
Include="Department,OnlineCourse,OnsiteCourse,StudentGrades.Person"
OnSelected=" CourseDetailsEntityDataSource_Selected">
<WhereParameters>
<asp:ControlParameter ControlID="CoursesGridView" Type="Int32"
Name="CourseID" PropertyName="SelectedValue" />
</WhereParameters>
</asp:EntityDataSource>
<asp:DetailsView ID="CourseDetailsView" runat="server"
AutoGenerateRows="False"
DataSourceID="CourseDetailsEntityDataSource">
<EmptyDataTemplate>
<p>
No course selected.</p>
</EmptyDataTemplate>
<Fields>
<asp:BoundField DataField="CourseID" HeaderText="ID" ReadOnly="True"
SortExpression="CourseID" />
<asp:BoundField DataField="Title" HeaderText="Title"
SortExpression="Title" />
<asp:BoundField DataField="Credits" HeaderText="Credits"
SortExpression="Credits" />
<asp:TemplateField HeaderText="Department">
<ItemTemplate>
<asp:Label ID="DetailsViewDepartmentLabel" runat="server"
Text='<%#Eval("Department.Name") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Location">
<ItemTemplate>
<asp:Label ID="LocationLabel" runat="server" Text='<%#
Eval("OnsiteCourse.Location") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="URL">
<ItemTemplate>
<asp:Label ID="URLLabel" runat="server" Text='<%#
Eval("OnlineCourse.URL") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Fields>
</asp:DetailsView>
</div>

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This markup creates an EntityDataSource control that’s bound to the Courses entity set. The Where property selects a course using the CourseID value of the selected row in the courses GridView control. The markup specifies a handler for the Selected event, which you’ll use later for displaying student grades, which is another level lower in the hierarchy.

In Instructors.aspx.cs, create the following stub for the CourseDetailsEntityDataSource_Selected method. (You’ll fill this stub out later in the tutorial; for now, you need it so that the page will compile and run.)

protected void CourseDetailsEntityDataSource_Selected(object sender,
EntityDataSourceSelectedEventArgs e)
{
}

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Run the page.

Initially there are no course details because no course is selected. Select an instructor who has a course assigned, and then select a course to see the details.

Using the EntityDataSource “Selected” Event to Display Related Data

Finally, you want to show all of the enrolled students and their grades for the selected course. To do this, you’ll use the Selected event of the EntityDataSource control bound to the course DetailsView.

In Instructors.aspx, add the following markup after the DetailsView control:

<h3>Student Grades</h3>
<asp:ListView ID="GradesListView" runat="server">
<EmptyDataTemplate>
<p>No student grades found.</p>
</EmptyDataTemplate>
<LayoutTemplate>
<table border="1" runat="server" id="itemPlaceholderContainer">
<tr runat="server">
<th runat="server">
Name
</th>
<th runat="server">
Grade
</th>
</tr>
<tr id="itemPlaceholder" runat="server">
</tr>
</table>
</LayoutTemplate>
<ItemTemplate>
<tr>
<td>
<asp:Label ID="StudentLastNameLabel" runat="server" Text='<%#
Eval("Person.LastName") %>' />,
<asp:Label ID="StudentFirstNameLabel" runat="server"
Text='<%#Eval("Person.FirstMidName") %>' />
</td>
<td>
<asp:Label ID="StudentGradeLabel" runat="server" Text='<%#
Eval("Grade") %>' />
</td>
</tr>
</ItemTemplate>
</asp:ListView>

Click here to view code as image

This markup creates a ListView control that displays a list of students and their grades for the selected course. No data source is specified because you’ll databind the control in code. The EmptyDataTemplate element provides a message to display when no course is selected—in that case, there are no students to display. The LayoutTemplate element creates an HTML table to display the list, and the ItemTemplate specifies the columns to display. The student ID and the student grade are from the StudentGrade entity, and the student name is from the Person entity that the Entity Framework makes available in the Person navigation property of the StudentGrade entity.

In Instructors.aspx.cs, replace the stubbed-out CourseDetailsEntityDataSource_Selected method with the following code:

protected void CourseDetailsEntityDataSource_Selected(object sender,
EntityDataSourceSelectedEventArgs e)
{
var course=e. Results. Cast<Course>(). FirstOrDefault();
if (course != null)
{
var studentGrades=course. StudentGrades. ToList();
GradesListView.DataSource=studentGrades;
GradesListView. DataBind();
}
}

Click here to view code as image

The event argument for this event provides the selected data in the form of a collection, which will have zero items if nothing is selected or one item if a Course entity is selected. If a Course entity is selected, the code uses the First method to convert the collection to a single object. It then gets StudentGrade entities from the navigation property, converts them to a collection, and binds the GradesListView control to the collection.

This is sufficient to display grades, but you want to make sure that the message in the empty data template is displayed the first time the page is displayed and whenever a course is not selected. To do that, create the following method, which you’ll call from two places:

private void ClearStudentGradesDataSource()
{
var emptyStudentGradesList = new List<StudentGrade>();
GradesListView.DataSource = emptyStudentGradesList;
GradesListView. DataBind();
}

Click here to view code as image

Call this new method from the Page_Load method to display the empty data template the first time the page is displayed. And call it from the InstructorsGridView_SelectedIndexChanged method because that event is raised when an instructor is selected, which means new courses are loaded into the courses GridView control and none is selected yet. Here are the two calls:

protected void Page_Load(object sender, EventArgs e)
{
if (! IsPostBack)
{
ClearStudentGradesDataSource();
}
}
protected void InstructorsGridView_SelectedIndexChanged(object sender,
EventArgs e)
{
ClearStudentGradesDataSource();
}

Click here to view code as image

Run the page.

Select an instructor that has a course assigned, and then select the course.

You have now seen a few ways to work with related data. In the following tutorial, you’ll learn how to add relationships between existing entities, how to remove relationships, and how to add a new entity that has a relationship to an existing entity.

Working with Related Data, Continued

In the previous tutorial you began to use the EntityDataSource control to work with related data. You displayed multiple levels of hierarchy and edited data in navigation properties. In this tutorial you’ll continue to work with related data by adding and deleting relationships and by adding a new entity that has a relationship to an existing entity.

You’ll create a page that adds courses that are assigned to departments. The departments already exist, and when you create a new course, at the same time you’ll establish a relationship between it and an existing department.

You’ll also create a page that works with a many-to-many relationship by assigning an instructor to a course (adding a relationship between two entities that you select) or removing an instructor from a course (removing a relationship between two entities that you select). In the database, adding a relationship between an instructor and a course results in a new row being added to the CourseInstructor association table; removing a relationship involves deleting a row from the CourseInstructor association table. However, you do this in the Entity Framework by setting navigation properties, without referring to the CourseInstructor table explicitly.

Adding an Entity with a Relationship to an Existing Entity

Create a new web page named CoursesAdd.aspx that uses the Site.Master master page, and add the following markup to the Content control named Content2:

<h2>Add Courses</h2>
<asp:EntityDataSource ID="CoursesEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="Courses"
EnableInsert="True" EnableDelete="True" >
</asp:EntityDataSource>
<asp:DetailsView ID="CoursesDetailsView" runat="server" AutoGenerateRows="False"
DataSourceID="CoursesEntityDataSource" DataKeyNames="CourseID"
DefaultMode="Insert" oniteminserting=" CoursesDetailsView_ItemInserting">
<Fields>
<asp:BoundField DataField="CourseID" HeaderText="ID" />
<asp:BoundField DataField="Title" HeaderText="Title" />
<asp:BoundField DataField="Credits" HeaderText="Credits" />
<asp:TemplateField HeaderText="Department">
<InsertItemTemplate>
<asp:EntityDataSource ID="DepartmentsEntityDataSource"
runat="server" ConnectionString="name=SchoolEntities"
DefaultContainerName="SchoolEntities" EnableDelete="True"
EnableFlattening="False"
EntitySetName="Departments" EntityTypeFilter="Department">
</asp:EntityDataSource>
<asp:DropDownList ID="DepartmentsDropDownList" runat="server"
DataSourceID="DepartmentsEntityDataSource"
DataTextField="Name" DataValueField="DepartmentID"
oninit=" DepartmentsDropDownList_Init">
</asp:DropDownList>
</InsertItemTemplate>
</asp:TemplateField>
<asp:CommandField ShowInsertButton="True" />
</Fields>
</asp:DetailsView>

Click here to view code as image

This markup creates an EntityDataSource control that selects courses, that enables inserting, and that specifies a handler for the Inserting event. You’ll use the handler to update the Department navigation property when a new Course entity is created.

The markup also creates a DetailsView control to use for adding new Course entities. The markup uses bound fields for Course entity properties. You have to enter the CourseID value because this is not a system-generated ID field. Instead, it’s a course number that must be specified manually when the course is created.

You use a template field for the Department navigation property because navigation properties cannot be used with BoundField controls. The template field provides a drop-down list to select the department. The drop-down list is bound to the Departments entity set by using Eval rather than Bind, again because you cannot directly bind navigation properties in order to update them. You specify a handler for the DropDownList control’s Init event so that you can store a reference to the control for use by the code that updates the DepartmentID foreign key.

In CoursesAdd.aspx.cs just after the partial-class declaration, add a class field to hold a reference to the DepartmentsDropDownList control:

private DropDownList departmentDropDownList;

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Add a handler for the DepartmentsDropDownList control’s Init event so that you can store a reference to the control. This lets you get the value the user has entered and use it to update the DepartmentID value of the Course entity.

protected void DepartmentsDropDownList_Init(object sender, EventArgs e)
{
departmentDropDownList=sender as DropDownList;
}

Click here to view code as image

Add a handler for the DetailsView control’s Inserting event:

protected void CoursesDetailsView_ItemInserting(object sender,
DetailsViewInsertEventArgs e)
{
var departmentID=Convert. ToInt32(departmentDropDownList. SelectedValue);
e. Values["DepartmentID"]=departmentID;
}

Click here to view code as image

When the user clicks Insert, the Inserting event is raised before the new record is inserted. The code in the handler gets the DepartmentID from the DropDownList control and uses it to set the value that will be used for the DepartmentID property of the Course entity.

The Entity Framework will take care of adding this course to the Courses navigation property of the associated Department entity. It also adds the department to the Department navigation property of the Course entity.

Run the page.

Enter an ID, a title, a number of credits, and select a department, then click Insert.

Run the Courses.aspx page, and select the same department to see the new course.

Working with Many-to-Many Relationships

The relationship between the Courses entity set and the People entity set is a many-to-many relationship. A Course entity has a navigation property named People that can contain zero, one, or more related Person entities (representing instructors assigned to teach that course). And a Person entity has a navigation property named Courses that can contain zero, one, or more related Course entities (representing courses that that instructor is assigned to teach). One instructor might teach multiple courses, and one course might be taught by multiple instructors. In this section of the walkthrough, you’ll add and remove relationships between Person and Course entities by updating the navigation properties of the related entities.

Create a new web page named InstructorsCourses.aspx that uses the Site.Master master page, and add the following markup to the Content control named Content2:

<h2>Assign Instructors to Courses or Remove from Courses</h2>
<br />
<asp:EntityDataSource ID="InstructorsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People"
Where="it.HireDate is not null" Select="it.LastName + ', ' + it.FirstMidName
AS Name, it.PersonID">
</asp:EntityDataSource>
Select an Instructor:
<asp:DropDownList ID="InstructorsDropDownList" runat="server"
DataSourceID="InstructorsEntityDataSource"
AutoPostBack="true" DataTextField="Name" DataValueField="PersonID"
OnSelectedIndexChanged=" InstructorsDropDownList_SelectedIndexChanged"
OnDataBound=" InstructorsDropDownList_DataBound">
</asp:DropDownList>
<h3>
Assign a Course</h3>
<br />
Select a Course:
<asp:DropDownList ID="UnassignedCoursesDropDownList" runat="server"
DataTextField="Title" DataValueField="CourseID">
</asp:DropDownList>
<br />
<asp:Button ID="AssignCourseButton" runat="server" Text="Assign"
OnClick=" AssignCourseButton_Click" />
<br />
<asp:Label ID="CourseAssignedLabel" runat="server" Visible="false"
Text="Assignment successful"></asp:Label>
<br />
<h3>
Remove a Course</h3>
<br />
Select a Course:
<asp:DropDownList ID="AssignedCoursesDropDownList" runat="server"
DataTextField="title" DataValueField="courseiD">
</asp:DropDownList>
<br />
<asp:Button ID="RemoveCourseButton" runat="server" Text="Remove"
OnClick=" RemoveCourseButton_Click" />
<br />
<asp:Label ID="CourseRemovedLabel" runat="server" Visible="false" Text="Removal
successful"></asp:Label>

Click here to view code as image

This markup creates an EntityDataSource control that retrieves the name and PersonID of Person entities for instructors. A DropDrownList control is bound to the EntityDataSource control. The DropDownList control specifies a handler for the DataBound event. You’ll use this handler to databind the two drop-down lists that display courses.

The markup also creates the following group of controls to use for assigning a course to the selected instructor:

• A DropDownList control for selecting a course to assign. This control will be populated with courses that are currently not assigned to the selected instructor.

• A Button control to initiate the assignment.

• A Label control to display an error message if the assignment fails.

Finally, the markup also creates a group of controls to use for removing a course from the selected instructor.

In InstructorsCourses.aspx.cs, add a using statement:

using ContosoUniversity.DAL;

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Add a method for populating the two drop-down lists that display courses:

private void PopulateDropDownLists()
{
using (var context = new SchoolEntities())
{
var allCourses = (from c in context.Courses
select c).ToList();

var instructorID =
Convert.ToInt32(InstructorsDropDownList.SelectedValue);
var instructor = (from p in context.People.Include("Courses")
where p.PersonID == instructorID
select p).First();

var assignedCourses = instructor.Courses.ToList();
var unassignedCourses =
allCourses.Except(assignedCourses.AsEnumerable()).ToList();

UnassignedCoursesDropDownList.DataSource = unassignedCourses;
UnassignedCoursesDropDownList.DataBind();
UnassignedCoursesDropDownList.Visible = true;

AssignedCoursesDropDownList.DataSource=assignedCourses;
AssignedCoursesDropDownList.DataBind();
AssignedCoursesDropDownList.Visible=true;
}
}

Click here to view code as image

This code gets all courses from the Courses entity set and gets the courses from the Courses navigation property of the Person entity for the selected instructor. It then determines which courses are assigned to that instructor and populates the drop-down lists accordingly.

Add a handler for the Assign button’s Click event:

protected void AssignCourseButton_Click(object sender, EventArgs e)
{
using (var context = new SchoolEntities())
{
var instructorID =
Convert.ToInt32(InstructorsDropDownList.SelectedValue);
var instructor =(from p in context.People
where p.PersonID == instructorID
select p).First();
var courseID =
Convert.ToInt32(UnassignedCoursesDropDownList.SelectedValue);
var course=(from c in context.Courses
where c.CourseID == courseID
select c).First();
instructor.Courses.Add(course);
try
{
context.SaveChanges();
PopulateDropDownLists();
CourseAssignedLabel.Text="Assignment successful.";
}
catch ( Exception)
{
CourseAssignedLabel.Text="Assignment unsuccessful.";
//Add code to log the error.
}
CourseAssignedLabel.Visible=true;
}
}

Click here to view code as image

This code gets the Person entity for the selected instructor, gets the Course entity for the selected course, and adds the selected course to the Courses navigation property of the instructor’s Person entity. It then saves the changes to the database and repopulates the drop-down lists so the results can be seen immediately.

Add a handler for the Remove button’s Click event:

protected void RemoveCourseButton_Click(object sender, EventArgs e)
{
using (var context =new SchoolEntities())
{
var instructorID =
Convert.ToInt32(InstructorsDropDownList.SelectedValue);
var instructor = (from p in context.People
where p.PersonID == instructorID
select p).First();
var courseID =
Convert.ToInt32(AssignedCoursesDropDownList.SelectedValue);
var courses =instructor.Courses;
var courseToRemove =new Course();
foreach (Course c in courses)
{
if (c.CourseID == courseID)
{
courseToRemove = c;
break;
}
}
try
{
courses.Remove(courseToRemove);
context.SaveChanges();
PopulateDropDownLists();
CourseRemovedLabel.Text ="Removal successful.";
}
catch (Exception)
{
CourseRemovedLabel.Text ="Removal unsuccessful.";
//Add code to log the error.
}
CourseRemovedLabel.Visible =true;
}
}

Click here to view code as image

This code gets the Person entity for the selected instructor, gets the Course entity for the selected course, and removes the selected course from the Person entity’s Courses navigation property. It then saves the changes to the database and repopulates the drop-down lists so the results can be seen immediately.

Add code to the Page_Load method that makes sure the error messages are not visible when there’s no error to report, and add handlers for the DataBound and SelectedIndexChanged events of the instructors drop-down list to populate the courses drop-down lists:

protected void Page_Load(object sender, EventArgs e)
{
CourseAssignedLabel. Visible=false;
CourseRemovedLabel. Visible=false;
}

protected void InstructorsDropDownList_DataBound(object sender, EventArgs e)
{
PopulateDropDownLists();
}

protected void InstructorsDropDownList_SelectedIndexChanged(object sender,
EventArgs e)
{
PopulateDropDownLists();
}

Click here to view code as image

Run the page.

Select an instructor. The Assign a Course drop-down list displays the courses that the instructor doesn’t teach, and the Remove a Course drop-down list displays the courses that the instructor is already assigned to. In the Assign a Course section, select a course and then click Assign. The course moves to the Remove a Course drop-down list. Select a course in the Remove a Course section and click Remove. The course moves to the Assign a Course drop-down list.

You have now seen some more ways to work with related data. In the following tutorial, you’ll learn how to use inheritance in the data model to improve the maintainability of your application.

Implementing Table-per-Hierarchy Inheritance

In the previous tutorial you worked with related data by adding and deleting relationships and by adding a new entity that had a relationship to an existing entity. This tutorial will show you how to implement inheritance in the data model.

In object-oriented programming, you can use inheritance to make it easier to work with related classes. For example, you could create Instructor and Student classes that derive from a Person base class. You can create the same kinds of inheritance structures among entities in the Entity Framework.

In this part of the tutorial, you won’t create any new web pages. Instead, you’ll add derived entities to the data model and modify existing pages to use the new entities.

Table-per-Hierarchy versus Table-per-Type Inheritance

A database can store information about related objects in one table or in multiple tables. For example, in the School database, the Person table includes information about both students and instructors in a single table. Some of the columns apply only to instructors (HireDate), some only to students (EnrollmentDate), and some to both (LastName, FirstName).

You can configure the Entity Framework to create Instructor and Student entities that inherit from the Person entity. This pattern of generating an entity inheritance structure from a single database table is called table-per-hierarchy (TPH) inheritance.

For courses, the School database uses a different pattern. Online courses and onsite courses are stored in separate tables, each of which has a foreign key that points to the Course table. Information common to both course types is stored only in the Course table.

You can configure the Entity Framework data model so that OnlineCourse and OnsiteCourse entities inherit from the Course entity. This pattern of generating an entity inheritance structure from separate tables for each type, with each separate table referring back to a table that stores data common to all types, is called table per type (TPT) inheritance.

TPH inheritance patterns generally deliver better performance in the Entity Framework than TPT inheritance patterns, because TPT patterns can result in complex join queries. This walkthrough demonstrates how to implement TPH inheritance. You’ll do that by performing the following steps:

• Create Instructor and Student entity types that derive from Person.

• Move properties that pertain to the derived entities from the Person entity to the derived entities.

• Set constraints on properties in the derived types.

• Make the Person entity an abstract entity.

• Map each derived entity to the Person table with a condition that specifies how to determine whether a Person row represents that derived type.

Adding Instructor and Student Entities

Open the SchoolModel.edmx file, right-click an unoccupied area in the designer, select Add, then select Entity.

In the Add Entity dialog box, name the entity Instructor and set its Base type option to Person.

Click OK. The designer creates an Instructor entity that derives from the Person entity. The new entity does not yet have any properties.

Repeat the procedure to create a Student entity that also derives from Person.

Only instructors have hire dates, so you need to move that property from the Person entity to the Instructor entity. In the Person entity, right-click the HireDate property and click Cut. Then right-click Properties in the Instructor entity and click Paste.

The hire date of an Instructor entity cannot be null. Right-click the HireDate property, click Properties, and then in the Properties window change Nullable to False.

Repeat the procedure to move the EnrollmentDate property from the Person entity to the Student entity. Make sure that you also set Nullable to False for the EnrollmentDate property.

Now that a Person entity has only the properties that are common to Instructor and Student entities (aside from navigation properties, which you’re not moving), the entity can only be used as a base entity in the inheritance structure. Therefore, you need to ensure that it’s never treated as an independent entity. Right-click the Person entity, select Properties, and then in the Properties window change the value of the Abstract property to True.

Mapping Instructor and Student Entities to the Person Table

Now you need to tell the Entity Framework how to differentiate between Instructor and Student entities in the database.

Right-click the Instructor entity and select Table Mapping. In the Mapping Details window, click Add a Table or View and select Person.

Click Add a Condition, and then select HireDate.

Change Operator to Is and Value / Property to Not Null.

Repeat the procedure for the Students entity, specifying that this entity maps to the Person table when the EnrollmentDate column is not null. Then save and close the data model.

Build the project in order to create the new entities as classes and make them available in the designer.

Using the Instructor and Student Entities

When you created the web pages that work with student and instructor data, you databound them to the Person entity set, and you filtered on the HireDate or EnrollmentDate property to restrict the returned data to students or instructors. However, now when you bind each data source control to the Person entity set, you can specify that only Student or Instructor entity types should be selected. Because the Entity Framework knows how to differentiate students and instructors in the Person entity set, you can remove the Where property settings you entered manually to do that.

In the Visual Studio Designer, you can specify the entity type that an EntityDataSource control should select in the EntityTypeFilter drop-down box of the Configure Data Source wizard, as shown in the following example.

And in the Properties window you can remove Where clause values that are no longer needed, as shown in the following example.

However, because you’ve changed the markup for EntityDataSource controls to use the ContextTypeName attribute, you cannot run the Configure Data Source wizard on EntityDataSource controls that you’ve already created. Therefore, you’ll make the required changes by changing markup instead.

Open the Students.aspx page. In the StudentsEntityDataSource control, remove the Where attribute and add an EntityTypeFilter="Student" attribute. The markup will now resemble the following example:

<asp:EntityDataSource ID="StudentsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People" EntityTypeFilter="Student"
Include="StudentGrades"
EnableDelete="True" EnableUpdate="True"
OrderBy="it.LastName" >
</asp:EntityDataSource>

Click here to view code as image

Setting the EntityTypeFilter attribute ensures that the EntityDataSource control will select only the specified entity type. If you wanted to retrieve both Student and Instructor entity types, you would not set this attribute. (You have the option of retrieving multiple entity types with one EntityDataSource control only if you’re using the control for read-only data access. If you’re using an EntityDataSource control to insert, update, or delete entities, and if the entity set it’s bound to can contain multiple types, you can only work with one entity type, and you have to set this attribute.)

Repeat the procedure for the SearchEntityDataSource control, except remove only the part of the Where attribute that selects Student entities instead of removing the property altogether. The opening tag of the control will now resemble the following example:

<asp:EntityDataSource ID="SearchEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People" EntityTypeFilter="Student"
Where="it.FirstMidName Like '%' + @StudentName + '%' or it.LastName Like '%'
+ @StudentName + '%'" >

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Run the page to verify that it still works as it did before.

Update the following pages that you created in earlier tutorials so that they use the new Student and Instructor entities instead of Person entities, then run them to verify that they work as they did before:

• In StudentsAdd.aspx, add EntityTypeFilter="Student" to the StudentsEntityDataSource control. The markup will now resemble the following example:

<asp:EntityDataSource ID="StudentsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People" EntityTypeFilter="Student"
EnableInsert="True"
</asp:EntityDataSource>

Click here to view code as image

• In About.aspx, add EntityTypeFilter="Student" to the StudentStatisticsEntityDataSource control and remove Where="it.EnrollmentDate is not null". The markup will now resemble the following example:

<asp:EntityDataSource ID="StudentStatisticsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People" EntityTypeFilter="Student"
Select="it.EnrollmentDate, Count(it.EnrollmentDate) AS NumberOfStudents"
OrderBy="it.EnrollmentDate" GroupBy="it.EnrollmentDate" >
</asp:EntityDataSource>

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• In Instructors.aspx and InstructorsCourses.aspx, add EntityTypeFilter="Instructor" to the InstructorsEntityDataSource control and remove Where="it.HireDate is not null". The markup in Instructors.aspx now resembles the following example:

<asp:EntityDataSource ID="InstructorsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="false"
EntitySetName="People" EntityTypeFilter="Instructor"
Include="OfficeAssignment"
EnableUpdate="True">
</asp:EntityDataSource>

Click here to view code as image

The markup in InstructorsCourses.aspx will now resemble the following example:

<asp:EntityDataSource ID="InstructorsEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="False"
EntitySetName="People" EntityTypeFilter="Instructor"
Select="it.LastName + ',' + it.FirstMidName AS Name, it.PersonID">
</asp:EntityDataSource>

Click here to view code as image

As a result of these changes, you’ve improved the Contoso University application’s maintainability in several ways. You’ve moved selection and validation logic out of the UI layer (.aspx markup) and made it an integral part of the data access layer. This helps to isolate your application code from changes that you might make in the future to the database schema or the data model. For example, you could decide that students might be hired as teachers’ aids and therefore would get a hire date. You could then add a new property to differentiate students from instructors and update the data model. No code in the web application would need to change except where you wanted to show a hire date for students. Another benefit of adding Instructor and Student entities is that your code is more readily understandable than when it referred to Person objects that were actually students or instructors.

You’ve now seen one way to implement an inheritance pattern in the Entity Framework. In the following tutorial, you’ll learn how to use stored procedures in order to have more control over how the Entity Framework accesses the database.

Using Stored Procedures

In the previous tutorial you implemented a table-per-hierarchy inheritance pattern. This tutorial will show you how to use stored procedures to gain more control over database access.

The Entity Framework lets you specify that it should use stored procedures for database access. For any entity type, you can specify a stored procedure to use for creating, updating, or deleting entities of that type. Then in the data model you can add references to stored procedures that you can use to perform tasks such as retrieving sets of entities.

Using stored procedures is a common requirement for database access. In some cases a database administrator may require that all database access go through stored procedures for security reasons. In other cases you may want to build business logic into some of the processes that the Entity Framework uses when it updates the database. For example, whenever an entity is deleted you might want to copy it to an archive database. Or whenever a row is updated you might want to write a row to a logging table that records who made the change. You can perform these kinds of tasks in a stored procedure that’s called whenever the Entity Framework deletes an entity or updates an entity.

As in the previous tutorial, you’ll not create any new pages. Instead, you’ll change the way the Entity Framework accesses the database for some of the pages you already created.

In this tutorial you’ll create stored procedures in the database for inserting Student and Instructor entities. You’ll add them to the data model, and you’ll specify that the Entity Framework should use them for adding Student and Instructor entities to the database. You’ll also create a stored procedure that you can use to retrieve Course entities.

Creating Stored Procedures in the Database

(If you’re using the School.mdf file from the project available for download with this tutorial, you can skip this section because the stored procedures already exist.)

In Server Explorer, expand School.mdf, right-click Stored Procedures, and select Add New Stored Procedure.

Copy the following SQL statements and paste them into the stored procedure window, replacing the skeleton stored procedure.

CREATE PROCEDURE [dbo].[InsertStudent]
@LastName nvarchar(50),
@FirstName nvarchar(50),
@EnrollmentDate datetime
AS
INSERT INTO dbo .Person (LastName,
FirstName,
EnrollmentDate)
VALUES (@LastName,
@FirstName,
@EnrollmentDate);
SELECT SCOPE_IDENTITY() as NewPersonID;

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Student entities have four properties: PersonID, LastName, FirstName, and EnrollmentDate. The database generates the ID value automatically, and the stored procedure accepts parameters for the other three. The stored procedure returns the value of the new row’s record key so that the Entity Framework can keep track of that in the version of the entity it keeps in memory.

Save and close the stored procedure window.

Create an InsertInstructor stored procedure in the same manner, using the following SQL statements:

CREATE PROCEDURE [dbo].[InsertInstructor]
@LastName nvarchar(50),
@FirstName nvarchar(50),
@HireDate datetime
AS
INSERT INTO dbo.Person (LastName,
FirstName,
HireDate)
VALUES (@LastName,
@FirstName,
@HireDate);
SELECT SCOPE_IDENTITY() as NewPersonID;

Click here to view code as image

Create Update stored procedures for Student and Instructor entities also. (The database already has a DeletePerson stored procedure which will work for both Instructor and Student entities.)

CREATE PROCEDURE [dbo].[UpdateStudent]
@PersonID int,
@LastName nvarchar(50),
@FirstName nvarchar(50),
@EnrollmentDate datetime
AS
UPDATE Person SET LastName=@LastName,
FirstName=@FirstName,
EnrollmentDate=@EnrollmentDate
WHERE PersonID=@PersonID;

CREATE PROCEDURE [dbo].[UpdateInstructor]
@PersonID int,
@LastName nvarchar(50),
@FirstName nvarchar(50),
@HireDate datetime
AS
UPDATE Person SET LastName=@LastName,
FirstName=@FirstName,
HireDate=@HireDate
WHERE PersonID=@PersonID;

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In this tutorial you’ll map all three functions -- insert, update, and delete -- for each entity type. The Entity Framework version 4 allows you to map just one or two of these functions to stored procedures without mapping the others, with one exception: if you map the update function but not the delete function, the Entity Framework will throw an exception when you attempt to delete an entity. In the Entity Framework version 3.5, you did not have this much flexibility in mapping stored procedures: if you mapped one function you were required to map all three.

To create a stored procedure that reads rather than updates data, create one that selects all Course entities, using the following SQL statements:

CREATE PROCEDURE [dbo].[GetCourses]
AS
SELECT CourseID, Title, Credits, DepartmentID FROM dbo.Course

Click here to view code as image

Adding the Stored Procedures to the Data Model

The stored procedures are now defined in the database, but they must be added to the data model to make them available to the Entity Framework. Open SchoolModel.edmx, right-click the design surface, and select Update Model from Database. In the Add tab of the Choose Your Database Objects dialog box, expand Stored Procedures, select the newly created stored procedures and the DeletePerson stored procedure, and then click Finish.

Mapping the Stored Procedures

In the data model designer, right-click the Student entity and select Stored Procedure Mapping.

The Mapping Details window appears, in which you can specify stored procedures that the Entity Framework should use for inserting, updating, and deleting entities of this type.

Set the Insert function to InsertStudent. The window shows a list of stored procedure parameters, each of which must be mapped to an entity property. Two of these are mapped automatically because the names are the same. There’s no entity property named FirstName, so you must manually select FirstMidName from a drop-down list that shows available entity properties. (This is because you changed the name of the FirstName property to FirstMidName in the first tutorial.)

In the same Mapping Details window, map the Update function to the UpdateStudent stored procedure (make sure you specify FirstMidName as the parameter value for FirstName, as you did for the Insert stored procedure) and the Delete function to the DeletePerson stored procedure.

Follow the same procedure to map the insert, update, and delete stored procedures for instructors to the Instructor entity.

For stored procedures that read rather than update data, you use the Model Browser window to map the stored procedure to the entity type it returns. In the data model designer, right-click the design surface and select Model Browser. Open the SchoolModel.Store node and then open the Stored Procedures node. Then right-click the GetCourses stored procedure and select Add Function Import.

In the Add Function Import dialog box, under Returns a Collection Of select Entities, and then select Course as the entity type returned. When you’re done, click OK. Save and close the .edmx file.

Using Insert, Update, and Delete Stored Procedures

Stored procedures to insert, update, and delete data are used by the Entity Framework automatically after you’ve added them to the data model and mapped them to the appropriate entities. You can now run the StudentsAdd.aspx page, and every time you create a new student, the Entity Framework will use the InsertStudent stored procedure to add the new row to the Student table.

Run the Students.aspx page and the new student appears in the list.

Change the name to verify that the update function works, and then delete the student to verify that the delete function works.

Using Select Stored Procedures

The Entity Framework does not automatically run stored procedures such as GetCourses, and you cannot use them with the EntityDataSource control. To use them, you call them from code.

Open the InstructorsCourses.aspx.cs file. The PopulateDropDownLists method uses a LINQ-to-Entities query to retrieve all course entities so that it can loop through the list and determine which ones an instructor is assigned to and which ones are unassigned:

var allCourses = (from c in context.Courses
select c).ToList();

Click here to view code as image

Replace this with the following code:

var allCourses = context.GetCourses();

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The page now uses the GetCourses stored procedure to retrieve the list of all courses. Run the page to verify that it works as it did before.

(Navigation properties of entities retrieved by a stored procedure might not be automatically populated with the data related to those entities, depending on ObjectContext default settings. For more information, see Loading Related Objects in the MSDN Library.)

In the next tutorial, you’ll learn how to use Dynamic Data functionality to make it easier to program and test data formatting and validation rules. Instead of specifying on each web page rules such as data format strings and whether or not a field is required, you can specify such rules in data model metadata and they’re automatically applied on every page.

Using Dynamic Data Functionality to Format and Validate Data

In the previous tutorial you implemented stored procedures. This tutorial will show you how Dynamic Data functionality can provide the following benefits:

• Fields are automatically formatted for display based on their data type.

• Fields are automatically validated based on their data type.

• You can add metadata to the data model to customize formatting and validation behavior. When you do this, you can add the formatting and validation rules in just one place, and they’re automatically applied everywhere you access the fields using Dynamic Data controls.

To see how this works, you’ll change the controls you use to display and edit fields in the existing Students.aspx page, and you’ll add formatting and validation metadata to the name and date fields of the Student entity type.

Using DynamicField and DynamicControl Controls

Open the Students.aspx page and in the StudentsGridView control replace the Name and Enrollment Date TemplateField elements with the following markup:

<asp:TemplateField HeaderText="Name" SortExpression="LastName">
<EditItemTemplate>
<asp:DynamicControl ID="LastNameTextBox" runat="server"
DataField="LastName" Mode="Edit" />
<asp:DynamicControl ID="FirstNameTextBox" runat="server"
DataField="FirstMidName" Mode="Edit" />
</EditItemTemplate>
<ItemTemplate>
<asp:DynamicControl ID="LastNameLabel" runat="server"
DataField="LastName" Mode="ReadOnly" />,
<asp:DynamicControl ID="FirstNameLabel" runat="server"
DataField="FirstMidName" Mode="ReadOnly" />
</ItemTemplate>
</asp:TemplateField>
<asp:DynamicField DataField="EnrollmentDate" HeaderText="Enrollment Date"
SortExpression="EnrollmentDate" />

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This markup uses DynamicControl controls in place of TextBox and Label controls in the student name template field, and it uses a DynamicField control for the enrollment date. No format strings are specified.

Add a ValidationSummary control after the StudentsGridView control.

<asp:ValidationSummary ID="StudentsValidationSummary" runat="server"
ShowSummary="true"
DisplayMode="BulletList" Style="color: Red" />

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In the SearchGridView control replace the markup for the Name and Enrollment Date columns as you did in the StudentsGridView control, except omit the EditItemTemplate element. The Columns element of the SearchGridView control now contains the following markup:

<asp:TemplateField HeaderText="Name" SortExpression="LastName">
<ItemTemplate>
<asp:DynamicControl ID="LastNameLabel" runat="server"
DataField="LastName" Mode="ReadOnly" />,
<asp:DynamicControl ID="FirstNameLabel" runat="server"
DataField="FirstMidName" Mode="ReadOnly" />
</ItemTemplate>
</asp:TemplateField>
<asp:DynamicField DataField="EnrollmentDate" HeaderText="Enrollment Date"
SortExpression="EnrollmentDate" />

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Open Students.aspx.cs and add the following using statement:

using ContosoUniversity.DAL;

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Add a handler for the page’s Init event:

protected void Page_Init(object sender, EventArgs e)
{
StudentsGridView.EnableDynamicData(typeof(Student));
SearchGridView.EnableDynamicData(typeof(Student));
}

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This code specifies that Dynamic Data will provide formatting and validation in these data-bound controls for fields of the Student entity. If you get an error message like the following example when you run the page, it typically means you’ve forgotten to call the EnableDynamicData method in Page_Init:

Could not determine a MetaTable. A MetaTable could not be determined for the data source ’StudentsEntityDataSource’ and one could not be inferred from the request URL.

Run the page.

In the Enrollment Date column, the time is displayed along with the date because the property type is DateTime. You’ll fix that later.

For now, notice that Dynamic Data automatically provides basic data validation. For example, click Edit, clear the date field, click Update, and you see that Dynamic Data automatically makes this a required field because the value is not nullable in the data model. The page displays an asterisk after the field and an error message in the ValidationSummary control:

You could omit the ValidationSummary control, because you can also hold the mouse pointer over the asterisk to see the error message:

Dynamic Data will also validate that data entered in the Enrollment Date field is a valid date:

As you can see, this is a generic error message. In the next section you’ll see how to customize messages as well as validation and formatting rules.

Adding Metadata to the Data Model

Typically, you want to customize the functionality provided by Dynamic Data. For example, you might change how data is displayed and the content of error messages. You typically also customize data validation rules to provide more functionality than what Dynamic Data provides automatically based on data types. To do this, you create partial classes that correspond to entity types.

In Solution Explorer, right-click the ContosoUniversity project, select Add Reference, and add a reference to System.ComponentModel.DataAnnotations.

In the DAL folder, create a new class file, name it Student.cs, and replace the template code in it with the following code.

using System;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;

namespace ContosoUniversity.DAL
{
[MetadataType(typeof(StudentMetadata))]
public partial class Student
{
}

public class StudentMetadata
{
[DisplayFormat(DataFormatString="{0:d}", ApplyFormatInEditMode=true)]
public DateTime EnrollmentDate { get; set; }

[StringLength(25, ErrorMessage = "First name must be 25 characters or less in
length.")]
[Required(ErrorMessage="First name is required.")]
public String FirstMidName { get; set; }

[StringLength(25, ErrorMessage = "Last name must be 25 characters or less in
length.")]
[Required(ErrorMessage = "Last name is required.")]
public String LastName { get; set; }
}
}

Click here to view code as image

This code creates a partial class for the Student entity. The MetadataType attribute applied to this partial class identifies the class that you’re using to specify metadata. The metadata class can have any name, but using the entity name plus “Metadata” is a common practice.

The attributes applied to properties in the metadata class specify formatting, validation, rules, and error messages. The attributes shown here will have the following results:

• EnrollmentDate will display as a date (without a time).

• Both name fields must be 25 characters or less in length, and a custom error message is provided.

• Both name fields are required, and a custom error message is provided.

Run the Students.aspx page again, and you see that the dates are now displayed without times:

Edit a row and try to clear the values in the name fields. The asterisks indicating field errors appear as soon as you leave a field, before you click Update. When you click Update, the page displays the error message text you specified.

Try to enter names that are longer than 25 characters, click Update, and the page displays the error message text you specified.

Now that you’ve set up these formatting and validation rules in the data model metadata, the rules will automatically be applied on every page that displays or allows changes to these fields, so long as you use DynamicControl or DynamicField controls. This reduces the amount of redundant code you have to write, which makes programming and testing easier, and it ensures that data formatting and validation are consistent throughout an application.

The ObjectDataSource Control

The EntityDataSource control enables you to create an application very quickly, but it typically requires you to keep a significant amount of business logic and data-access logic in your .aspx pages. If you expect your application to grow in complexity and to require ongoing maintenance, you can invest more development time up front in order to create an n-tier or layered application structure that’s more maintainable. To implement this architecture, you separate the presentation layer from the business logic layer (BLL) and the data access layer (DAL). One way to implement this structure is to use the ObjectDataSource control instead of the EntityDataSource control. When you use the ObjectDataSource control, you implement your own data-access code and then invoke it in .aspx pages using a control that has many of the same features as other data-source controls. This lets you combine the advantages of an n-tier approach with the benefits of using a Web Forms control for data access.

The ObjectDataSource control gives you more flexibility in other ways as well. Because you write your own data-access code, it’s easier to do more than just read, insert, update, or delete a specific entity type, which are the tasks that the EntityDataSource control is designed to perform. For example, you can perform logging every time an entity is updated, archive data whenever an entity is deleted, or automatically check and update related data as needed when inserting a row with a foreign key value.

Business Logic and Repository Classes

An ObjectDataSource control works by invoking a class that you create. The class includes methods that retrieve and update data, and you provide the names of those methods to the ObjectDataSource control in markup. During rendering or postback processing, the ObjectDataSource calls the methods that you’ve specified.

Besides basic CRUD operations, the class that you create to use with the ObjectDataSource control might need to execute business logic when the ObjectDataSource reads or updates data. For example, when you update a department, you might need to validate that no other departments have the same administrator because one person cannot be administrator of more than one department.

In some ObjectDataSource documentation, such as the ObjectDataSource Class overview, the control calls a class referred to as a business object that includes both business logic and data-access logic. In this tutorial you will create separate classes for business logic and for data-access logic. The class that encapsulates data-access logic is called a repository. The business logic class includes both business-logic methods and data-access methods, but the data-access methods call the repository to perform data-access tasks.

You will also create an abstraction layer between your BLL and DAL that facilitates automated unit testing of the BLL. This abstraction layer is implemented by creating an interface and using the interface when you instantiate the repository in the business-logic class. This makes it possible for you to provide the business-logic class with a reference to any object that implements the repository interface. For normal operation, you provide a repository object that works with the Entity Framework. For testing, you provide a repository object that works with data stored in a way that you can easily manipulate, such as class variables defined as collections.

The following illustration shows the difference between a business-logic class that includes data-access logic without a repository and one that uses a repository.

You will begin by creating web pages in which the ObjectDataSource control is bound directly to a repository because it only performs basic data-access tasks. In the next tutorial you will create a business logic class with validation logic and bind the ObjectDataSource control to that class instead of to the repository class. You will also create unit tests for the validation logic. In the third tutorial in this series you will add sorting and filtering functionality to the application.

The pages you create in this tutorial work with the Departments entity set of the data model that you created in previous tutorials in this series.

Updating the Database and the Data Model

You will begin this tutorial by making two changes to the database, both of which require corresponding changes to the data model that you created earlier. In one of the earlier tutorials, you made changes in the designer manually to synchronize the data model with the database after a database change. In this tutorial, you will use the designer’s Update Model From Database tool to update the data model automatically.

Adding a Relationship to the Database

In Visual Studio, open the Contoso University web application you created in the previous tutorials, and then open the SchoolDiagram database diagram.

If you look at the Department table in the database diagram, you will see that it has an Administrator column. This column is a foreign key to the Person table, but no foreign key relationship is defined in the database. You need to create the relationship and update the data model so that the Entity Framework can automatically handle this relationship.

In the database diagram, right-click the Department table, and select Relationships.

In the Foreign Key Relationships box click Add, then click the ellipsis for Tables and Columns Specification.

In the Tables and Columns dialog box, set the primary key table and field to Person and PersonID, and set the foreign key table and field to Department and Administrator. (When you do this, the relationship name will change from FK_Department_Department to FK_Department_Person.)

Click OK in the Tables and Columns box, click Close in the Foreign Key Relationships box, and save the changes. If you’re asked if you want to save the Person and Department tables, click Yes.

Note If you’ve deleted Person rows that correspond to data that’s already in the Administrator column, you will not be able to save this change. In that case, use the table editor in Server Explorer to make sure that the Administrator value in every Department row contains the ID of a record that actually exists in the Person table.

After you save the change, you will not be able to delete a row from the Person table if that person is a department administrator. In a production application, you would provide a specific error message when a database constraint prevents a deletion, or you would specify a cascading delete. For an example of how to specify a cascading delete, see The Entity Framework and ASP.NET – Getting Started Part 2.

Adding a View to the Database

In the new Departments.aspx page that you will be creating, you want to provide a drop-down list of instructors, with names in “last, first” format so that users can select department administrators. To make it easier to do that, you will create a view in the database. The view will consist of just the data needed by the drop-down list: the full name (properly formatted) and the record key.

In Server Explorer, expand School.mdf, right-click the Views folder, and select Add New View.

Click Close when the Add Table dialog box appears, and paste the following SQL statement into the SQL pane:

SELECT    LastName + ',' + FirstName AS FullName, PersonID
FROM      dbo.Person
WHERE     (HireDate IS NOT NULL)

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Save the view as vInstructorName.

Updating the Data Model

In the DAL folder, open the SchoolModel.edmx file, right-click the design surface, and select Update Model from Database.

In the Choose Your Database Objects dialog box, select the Add tab and select the view you just created.

Click Finish.

In the designer, you see that the tool created a vInstructorName entity and a new association between the Department and Person entities.

Note In the Output and Error List windows you might see a warning message informing you that the tool automatically created a primary key for the new vInstructorName view. This is expected behavior.

When you refer to the new vInstructorName entity in code, you don’t want to use the database convention of prefixing a lower-case “v” to it. Therefore, you will rename the entity and entity set in the model.

Open the Model Browser. You see vInstructorName listed as an entity type and a view.

Under SchoolModel (not SchoolModel.Store), right-click vInstructorName and select Properties. In the Properties window, change the Name property to “InstructorName” and change the Entity Set Name property to “InstructorNames”.

Save and close the data model, and then rebuild the project.

Using a Repository Class and an ObjectDataSource Control

Create a new class file in the DAL folder, name it SchoolRepository.cs, and replace the existing code with the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using ContosoUniversity.DAL;

namespace ContosoUniversity.DAL
{
public class SchoolRepository : IDisposable
{
private SchoolEntities context = new SchoolEntities();

public IEnumerable<Department> GetDepartments()
{
return context.Departments.Include("Person").ToList();

}

private bool disposedValue = false;

protected virtual void Dispose(bool disposing)
{
if (!this.disposedValue)
{
if (disposing)
{
context.Dispose();
}
}
this.disposedValue = true;
}

public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}

}
}

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This code provides a single GetDepartments method that returns all of the entities in the Departments entity set. Because you know that you will be accessing the Person navigation property for every row returned, you specify eager loading for that property by using the Include method. The class also implements the IDisposable interface to ensure that the database connection is released when the object is disposed.

Note A common practice is to create a repository class for each entity type. In this tutorial, one repository class for multiple entity types is used. For more information about the repository pattern, see the posts in the Entity Framework team’s blog and Julie Lerman’s blog.

The GetDepartments method returns an IEnumerable object rather than an IQueryable object in order to ensure that the returned collection is usable even after the repository object itself is disposed. An IQueryable object can cause database access whenever it’s accessed, but the repository object might be disposed by the time a databound control attempts to render the data. You could return another collection type, such as an IList object instead of an IEnumerable object. However, returning an IEnumerable object ensures that you can perform typical read-only list processing tasks such as foreach loops and LINQ queries, but you cannot add to or remove items in the collection, which might imply that such changes would be persisted to the database.

Create a Departments.aspx page that uses the Site.Master master page, and add the following markup in the Content control named Content2:

<h2>Departments</h2>
<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.DAL.SchoolRepository"
DataObjectTypeName="ContosoUniversity.DAL.Department"
SelectMethod="GetDepartments" >
</asp:ObjectDataSource>
<asp:GridViewID="DepartmentsGridView" runat="server" AutoGenerateColumns="False"
DataSourceID="DepartmentsObjectDataSource" >
<Columns>
<asp:CommandField ShowEditButton="True" ShowDeleteButton="True"
ItemStyle-VerticalAlign="Top">
</asp:CommandField>
<asp:DynamicField DataField="Name" HeaderText="Name"
SortExpression="Name" ItemStyle-VerticalAlign="Top" />
<asp:DynamicField DataField="Budget" HeaderText="Budget"
SortExpression="Budget" ItemStyle-VerticalAlign="Top" />
<asp:DynamicField DataField="StartDate" HeaderText="Start Date"
ItemStyle-VerticalAlign="Top" />
<asp:TemplateField HeaderText="Administrator"
SortExpression="Person.LastName" ItemStyle-VerticalAlign="Top" >
<ItemTemplate>
<asp:Label ID="AdministratorLastNameLabel" runat="server"
Text='<%# Eval("Person.LastName") %>'></asp:Label>,
<asp:Label ID="AdministratorFirstNameLabel" runat="server"
Text='<%# Eval("Person.FirstMidName") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

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This markup creates an ObjectDataSource control that uses the repository class you just created, and a GridView control to display the data. The GridView control specifies Edit and Delete commands, but you haven’t added code to support them yet.

Several columns use DynamicField controls so that you can take advantage of automatic data formatting and validation functionality. For these to work, you will have to call the EnableDynamicData method in the Page_Init event handler. (DynamicControl controls are not used in the Administrator field because they don’t work with navigation properties.)

The Vertical-Align="Top" attributes will become important later when you add a column that has a nested GridView control to the grid.

Open the Departments.aspx.cs file and add the following using statement:

using ContosoUniversity.DAL;

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Then add the following handler for the page’s Init event:

protected void Page_Init(object sender, EventArgs e)
{
DepartmentsGridView.EnableDynamicData(typeof(Department));
}

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In the DAL folder, create a new class file named Department.cs and replace the existing code with the following code:

using System;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;

namespace ContosoUniversity.DAL
{
[MetadataType(typeof(DepartmentMetaData))]
public partial class Department
{
}

public class DepartmentMetaData
{
[DataType(DataType.Currency)]
[Range(0, 1000000, ErrorMessage = "Budget must be less than $1,000,000.00" )]
public Decimal Budget { get; set; }

[DisplayFormat(DataFormatString="{0:d}" ,ApplyFormatInEditMode=true)]
public DateTime StartDate { get; set; }

}
}

Click here to view code as image

This code adds metadata to the data model. It specifies that the Budget property of the Department entity actually represents currency although its data type is Decimal, and it specifies that the value must be between 0 and $1,000,000.00. It also specifies that the StartDate property should be formatted as a date in the format mm/dd/yyyy.

Run the Departments.aspx page.

Notice that although you did not specify a format string in the Departments.aspx page markup for the Budget or Start Date columns, default currency and date formatting has been applied to them by the DynamicField controls, using the metadata that you supplied in the Department.cs file.

Adding Insert and Delete Functionality

Open SchoolRepository.cs, add the following code in order to create an Insert method and a Delete method. The code also includes a method named GenerateDepartmentID that calculates the next available record key value for use by the Insert method. This is required because the database is not configured to calculate this automatically for the Department table.

public void InsertDepartment(Department department)
{
try
{
department.DepartmentID = GenerateDepartmentID();
context.Departments.AddObject(department);
context.SaveChanges();
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

public void DeleteDepartment(Department department)
{
try
{
context.Departments.Attach(department);
context.Departments.DeleteObject(department);
context.SaveChanges();
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

private Int32 GenerateDepartmentID()
{
Int32 maxDepartmentID = 0;
var department = (from d in GetDepartments()

orderby d.DepartmentID descending
select d).FirstOrDefault();
if (department != null)
{
maxDepartmentID = department.DepartmentID + 1;
}
return maxDepartmentID;
}

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The Attach Method

The DeleteDepartment method calls the Attach method in order to re-establish the link that’s maintained in the object context’s object state manager between the entity in memory and the database row it represents. This must occur before the method calls the SaveChanges method.

The term object context refers to the Entity Framework class that derives from the ObjectContext class that you use to access your entity sets and entities. In the code for this project, the class is named SchoolEntities, and an instance of it is always named context. The object context’s object state manager is a class that derives from the ObjectStateManager class. The object contact uses the object state manager to store entity objects and to keep track of whether each one is in sync with its corresponding table row or rows in the database.

When you read an entity, the object context stores it in the object state manager and keeps track of whether that representation of the object is in sync with the database. For example, if you change a property value, a flag is set to indicate that the property you changed is no longer in sync with the database. Then when you call the SaveChanges method, the object context knows what to do in the database because the object state manager knows exactly what’s different between the current state of the entity and the state of the database.

However, this process typically does not work in a web application, because the object context instance that reads an entity, along with everything in its object state manager, is disposed after a page is rendered. The object context instance that must apply changes is a new one that’s instantiated for postback processing. In the case of the DeleteDepartment method, the ObjectDataSource control re-creates the original version of the entity for you from values in view state, but this re-created Department entity does not exist in the object state manager. If you called the DeleteObject method on this re-created entity, the call would fail because the object context does not know whether the entity is in sync with the database. However, calling the Attach method re-establishes the same tracking between the re-created entity and the values in the database that was originally done automatically when the entity was read in an earlier instance of the object context.

There are times when you don’t want the object context to track entities in the object state manager, and you can set flags to prevent it from doing that. Examples of this are shown in later tutorials in this series.

The SaveChanges Method

This simple repository class illustrates basic principles of how to perform CRUD operations. In this example, the SaveChanges method is called immediately after each update. In a production application you might want to call the SaveChanges method from a separate method to give you more control over when the database is updated. (At the end of the next tutorial you will find a link to a white paper that discusses the unit of work pattern which is one approach to coordinating related updates.) Notice also that in the example, the DeleteDepartment method does not include code for handling concurrency conflicts; code to do that will be added in a later tutorial in this series.

Retrieving Instructor Names to Select When Inserting

Users must be able to select an administrator from a list of instructors in a drop-down list when creating new departments. Therefore, add the following code to SchoolRepository.cs to create a method to retrieve the list of instructors using the view that you created earlier:

public IEnumerable<InstructorName> GetInstructorNames()
{
return context.InstructorNames.OrderBy("it.FullName").ToList();
}

Click here to view code as image

Creating a Page for Inserting Departments

Create a DepartmentsAdd.aspx page that uses the Site.Master page, and add the following markup in the Content control named Content2:

<h2>Departments</h2>
<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.DAL.SchoolRepository"
DataObjectTypeName="ContosoUniversity.DAL.Department"
InsertMethod="InsertDepartment" >
</asp:ObjectDataSource>
<asp:DetailsView ID="DepartmentsDetailsView" runat="server"
DataSourceID="DepartmentsObjectDataSource" AutoGenerateRows="False"
DefaultMode="Insert" OnItemInserting="DepartmentsDetailsView_ItemInserting" >
<Fields>
<asp:DynamicField DataField="Name" HeaderText="Name" />
<asp:DynamicField DataField="Budget" HeaderText="Budget" />
<asp:DynamicField DataField="StartDate" HeaderText="Start Date" />
<asp:TemplateField HeaderText="Administrator" >
<InsertItemTemplate>
<asp:ObjectDataSource ID="InstructorsObjectDataSource"
runat="server"
TypeName="ContosoUniversity.DAL.SchoolRepository"
DataObjectTypeName="ContosoUniversity.DAL.InstructorName"
SelectMethod="GetInstructorNames" >
</asp:ObjectDataSource>
<asp:DropDownList ID="InstructorsDropDownList" runat="server"
DataSourceID="InstructorsObjectDataSource"
DataTextField="FullName" DataValueField="PersonID"
OnInit="DepartmentsDropDownList_Init">
</asp:DropDownList>
</InsertItemTemplate>
</asp:TemplateField>
<asp:CommandField ShowInsertButton="True" />
</Fields>
</asp:DetailsView>
<asp:ValidationSummary ID="DepartmentsValidationSummary" runat="server"
ShowSummary="true" DisplayMode="BulletList" />

Click here to view code as image

This markup creates two ObjectDataSource controls, one for inserting new Department entities and one for retrieving instructor names for the DropDownList control that’s used for selecting department administrators. The markup creates a DetailsView control for entering new departments, and it specifies a handler for the control’s ItemInserting event so that you can set the Administrator foreign key value. At the end is a ValidationSummary control to display error messages.

Open DepartmentsAdd.aspx.cs and add the following using statement:

using ContosoUniversity.DAL;

Click here to view code as image

Add the following class variable and methods:

private DropDownList administratorsDropDownList;

protected void Page_Init(object sender, EventArgs e)
{
DepartmentsDetailsView.EnableDynamicData(typeof(Department));
}

protected void DepartmentsDropDownList_Init(object sender, EventArgs e)
{
administratorsDropDownList = sender as DropDownList;
}

protected void DepartmentsDetailsView_ItemInserting(object sender,
DetailsViewInsertEventArgs e)
{
e.Values["Administrator" ] = administratorsDropDownList.SelectedValue;
}

Click here to view code as image

The Page_Init method enables Dynamic Data functionality. The handler for the DropDownList control’s Init event saves a reference to the control, and the handler for the DetailsView control’s Inserting event uses that reference to get the PersonID value of the selected instructor and update the Administrator foreign key property of the Department entity.

Run the page, add information for a new department, and then click the Insert link.

Enter values for another new department. Enter a number greater than 1,000,000.00 in the Budget field and tab to the next field. An asterisk appears in the field, and if you hold the mouse pointer over it, you can see the error message that you entered in the metadata for that field.

Click Insert, and you see the error message displayed by the ValidationSummary control at the bottom of the page.

Next, close the browser and open the Departments.aspx page. Add delete capability to the Departments.aspx page by adding a DeleteMethod attribute to the ObjectDataSource control, and a DataKeyNames attribute to the GridView control. The opening tags for these controls will now resemble the following example:

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.DAL.SchoolRepository"
DataObjectTypeName="ContosoUniversity.DAL.Department"
SelectMethod="GetDepartments"
DeleteMethod="DeleteDepartment" >

<asp:GridView ID="DepartmentsGridView" runat="server" AutoGenerateColumns="False"
DataSourceID="DepartmentsObjectDataSource" DataKeyNames="DepartmentID" >

Click here to view code as image

Run the page.

Delete the department you added when you ran the DepartmentsAdd.aspx page.

Adding Update Functionality

Open SchoolRepository.cs and add the following Update method:

public void UpdateDepartment(Department department, Department
origDepartment)
{
try
{
context.Departments.Attach(origDepartment);
context.ApplyCurrentValues("Departments",department);
context.SaveChanges();
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

Click here to view code as image

When you click Update in the Departments.aspx page, the ObjectDataSource control creates two Department entities to pass to the UpdateDepartment method. One contains the original values that have been stored in view state, and the other contains the new values that were entered in the GridView control. The code in the UpdateDepartment method passes the Department entity that has the original values to the Attach method in order to establish the tracking between the entity and what’s in the database. Then the code passes the Department entity that has the new values to the ApplyCurrentValues method. The object context compares the old and new values. If a new value is different from an old value, the object context changes the property value. The SaveChanges method then updates only the changed columns in the database. (However, if the update function for this entity were mapped to a stored procedure, the entire row would be updated regardless of which columns were changed.)

Open the Departments.aspx file and add the following attributes to the DepartmentsObjectDataSource control:

• UpdateMethod="UpdateDepartment"

• ConflictDetection="CompareAllValues"
This causes old values to be stored in view state so that they can be compared with the new values in the Update method.

• OldValuesParameterFormatString="orig{0}"
This informs the control that the name of the original values parameter is origDepartment.

The markup for the opening tag of the ObjectDataSource control now resembles the following example:

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.DAL.SchoolRepository"
DataObjectTypeName="ContosoUniversity.DAL.Department"
SelectMethod="GetDepartments" DeleteMethod="DeleteDepartment"
UpdateMethod="UpdateDepartment"
ConflictDetection="CompareAllValues"
OldValuesParameterFormatString="orig{0}" >

Click here to view code as image

Add an OnRowUpdating="DepartmentsGridView_RowUpdating" attribute to the GridView control. You will use this to set the Administrator property value based on the row the user selects in a drop-down list. The GridView opening tag now resembles the following example:

<asp:GridView ID="DepartmentsGridView" runat="server" AutoGenerateColumns="False"
DataSourceID="DepartmentsObjectDataSource" DataKeyNames="DepartmentID"
OnRowUpdating="DepartmentsGridView_RowUpdating">

Click here to view code as image

Add an EditItemTemplate control for the Administrator column to the GridView control, immediately after the ItemTemplate control for that column:

<EditItemTemplate>
<asp:ObjectDataSource ID="InstructorsObjectDataSource"
runat="server" DataObjectTypeName="ContosoUniversity.DAL.InstructorName"
SelectMethod="GetInstructorNames"
TypeName="ContosoUniversity.DAL.SchoolRepository">
</asp:ObjectDataSource>
<asp:DropDownList ID="InstructorsDropDownList" runat="server"
DataSourceID="InstructorsObjectDataSource"
SelectedValue='<%# Eval("Administrator")  %>'
DataTextField="FullName" DataValueField="PersonID"
OnInit="DepartmentsDropDownList_Init" >
</asp:DropDownList>
</EditItemTemplate>

Click here to view code as image

This EditItemTemplate control is similar to the InsertItemTemplate control in the DepartmentsAdd.aspx page. The difference is that the initial value of the control is set using the SelectedValue attribute.

Before the GridView control, add a ValidationSummary control as you did in the DepartmentsAdd.aspx page.

<asp:ValidationSummary ID="DepartmentsValidationSummary" runat="server"
ShowSummary="true" DisplayMode="BulletList" />

Click here to view code as image

Open Departments.aspx.cs and immediately after the partial-class declaration, add the following code to create a private field to reference the DropDownList control:

private DropDownList administratorsDropDownList;

Click here to view code as image

Then add handlers for the DropDownList control’s Init event and the GridView control’s RowUpdating event:

protected void DepartmentsDropDownList_Init(object sender, EventArgs e)
{
administratorsDropDownList = sender as DropDownList;
}

protected void DepartmentsGridView_RowUpdating(object sender,
GridViewUpdateEventArgs e)
{
e.NewValues["Administrator"] = administratorsDropDownList.SelectedValue;
}

Click here to view code as image

The handler for the Init event saves a reference to the DropDownList control in the class field. The handler for the RowUpdating event uses the reference to get the value the user entered and apply it to the Administrator property of the Department entity.

Use the DepartmentsAdd.aspx page to add a new department, then run the Departments.aspx page and click Edit on the row that you added.

Note You will not be able to edit rows that you did not add (that is, that were already in the database), because of invalid data in the database; the administrators for the rows that were created with the database are students. If you try to edit one of them, you will get an error page that reports an error like ’InstructorsDropDownList’ has a SelectedValue which is invalid because it does not exist in the list of items.

If you enter an invalid Budget amount and then click Update, you see the same asterisk and error message that you saw in the Departments.aspx page.

Change a field value or select a different administrator and click Update. The change is displayed.

This completes the introduction to using the ObjectDataSource control for basic CRUD (create, read, update, delete) operations with the Entity Framework. You’ve built a simple n-tier application, but the business-logic layer is still tightly coupled to the data-access layer, which complicates automated unit testing. In the following tutorial you’ll see how to implement the repository pattern to facilitate unit testing.

Adding a Business Logic Layer and Unit Tests

In the previous tutorial you created an n-tier web application using the Entity Framework and the ObjectDataSource control. This tutorial shows how to add business logic while keeping the business-logic layer (BLL) and the data-access layer (DAL) separate, and it shows how to create automated unit tests for the BLL.

In this tutorial you’ll complete the following tasks:

• Create a repository interface that declares the data-access methods you need.

• Implement the repository interface in the repository class.

• Create a business-logic class that calls the repository class to perform data-access functions.

• Connect the ObjectDataSource control to the business-logic class instead of to the repository class.

• Create a unit-test project and a repository class that uses in-memory collections for its data store.

• Create a unit test for business logic that you want to add to the business-logic class, then run the test and see it fail.

• Implement the business logic in the business-logic class, then re-run the unit test and see it pass.

You’ll work with the Departments.aspx and DepartmentsAdd.aspx pages that you created in the previous tutorial.

Creating a Repository Interface

You’ll begin by creating the repository interface.

In the DAL folder, create a new class file, name it ISchoolRepository.cs, and replace the existing code with the following code:

using System;
using System.Collections.Generic;

namespace ContosoUniversity.DAL
{
public interface ISchoolRepository : IDisposable
{
IEnumerable<Department> GetDepartments();
void InsertDepartment(Department department);
void DeleteDepartment(Department department);
void UpdateDepartment(Department department, Department origDepartment);
IEnumerable<InstructorName> GetInstructorNames();
}
}

Click here to view code as image

The interface defines one method for each of the CRUD (create, read, update, delete) methods that you created in the repository class.

In the SchoolRepository class in SchoolRepository.cs, indicate that this class implements the ISchoolRepository interface:

public class SchoolRepository : IDisposable, ISchoolRepository

Click here to view code as image

Creating a Business-Logic Class

Next, you’ll create the business-logic class. You do this so that you can add business logic that will be executed by the ObjectDataSource control, although you will not do that yet. For now, the new business-logic class will only perform the same CRUD operations that the repository does.

Create a new folder and name it BLL. (In a real-world application, the business-logic layer would typically be implemented as a class library — a separate project — but to keep this tutorial simple, BLL classes will be kept in a project folder.)

In the BLL folder, create a new class file, name it SchoolBL.cs, and replace the existing code with the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using ContosoUniversity.DAL;

namespace ContosoUniversity.BLL
{
public class SchoolBL : IDisposable
{
private ISchoolRepository schoolRepository;

public SchoolBL()
{
this.schoolRepository = new SchoolRepository();
}

public SchoolBL(ISchoolRepository schoolRepository)
{
this.schoolRepository = schoolRepository;
}

public IEnumerable<Department> GetDepartments()
{
return schoolRepository.GetDepartments();
}

public void InsertDepartment(Department department)
{
try
{
schoolRepository.InsertDepartment(department);
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

public void DeleteDepartment(Department department)
{
try
{
schoolRepository.DeleteDepartment(department);
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

public void UpdateDepartment(Department department, Department
origDepartment)
{
try
{
schoolRepository.UpdateDepartment(department, origDepartment);
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,

//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}

}

public IEnumerable<InstructorName> GetInstructorNames()
{
return schoolRepository.GetInstructorNames();
}

private bool disposedValue = false;

protected virtual void Dispose(bool disposing)
{
if (!this.disposedValue)
{
if (disposing)
{
schoolRepository.Dispose();
}
}
this.disposedValue = true;
}

public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}

}
}

Click here to view code as image

This code creates the same CRUD methods you saw earlier in the repository class, but instead of accessing the Entity Framework methods directly, it calls the repository class methods.

The class variable that holds a reference to the repository class is defined as an interface type, and the code that instantiates the repository class is contained in two constructors. The parameterless constructor will be used by the ObjectDataSource control. It creates an instance of the SchoolRepository class that you created earlier. The other constructor allows whatever code that instantiates the business-logic class to pass in any object that implements the repository interface.

The CRUD methods that call the repository class and the two constructors make it possible to use the business-logic class with whatever back-end data store you choose. The business-logic class does not need to be aware of how the class that it’s calling persists the data. (This is often called persistence ignorance.) This facilitates unit testing, because you can connect the business-logic class to a repository implementation that uses something as simple as in-memory List collections to store data.

Note Technically, the entity objects are still not persistence-ignorant, because they’re instantiated from classes that inherit from the Entity Framework’s EntityObject class. For complete persistence ignorance, you can use plain old CLR objects, or POCOs, in place of objects that inherit from the EntityObject class. Using POCOs is beyond the scope of this tutorial. For more information, see Testability and Entity Framework 4.0 on the MSDN website.)

Now you can connect the ObjectDataSource controls to the business-logic class instead of to the repository and verify that everything works as it did before.

In Departments.aspx and DepartmentsAdd.aspx, change each occurrence of TypeName="ContosoUniversity.DAL.SchoolRepository" to TypeName="ContosoUniversity.BLL.SchoolBL". (There are four instances in all.)

Run the Departments.aspx and DepartmentsAdd.aspx pages to verify that they still work as they did before.

Creating a Unit-Test Project and Repository Implementation

Add a new project to the solution using the Test Project template, and name it ContosoUniversity.Tests.

In the test project, add a reference to System.Data.Entity and add a project reference to the ContosoUniversity project.

You can now create the repository class that you’ll use with unit tests. The data store for this repository will be within the class.

In the test project, create a new class file, name it MockSchoolRepository.cs, and replace the existing code with the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using ContosoUniversity.DAL;
using ContosoUniversity.BLL;

namespace ContosoUniversity.Tests
{
class MockSchoolRepository : ISchoolRepository, IDisposable
{
List<Department> departments = new List<Department>();
List<InstructorName> instructors = new List<InstructorName>();

public IEnumerable<Department> GetDepartments()
{
return departments;
}

public void InsertDepartment(Department department)
{
departments.Add(department);
}

public void DeleteDepartment(Department department)
{
departments.Remove(department);
}

public void UpdateDepartment(Department department, Department
origDepartment)
{
departments.Remove(origDepartment);
departments.Add(department);
}

public IEnumerable<InstructorName> GetInstructorNames()
{
return instructors;
}

public void Dispose()
{

}
}
}

Click here to view code as image

This repository class has the same CRUD methods as the one that accesses the Entity Framework directly, but they work with List collections in memory instead of with a database. This makes it easier for a test class to set up and validate unit tests for the business-logic class.

Creating Unit Tests

The Test project template created a stub unit test class for you, and your next task is to modify this class by adding unit test methods to it for business logic that you want to add to the business-logic class.

At Contoso University, any individual instructor can only be the administrator of a single department, and you need to add business logic to enforce this rule. You will start by adding tests and running the tests to see them fail. You’ll then add the code and rerun the tests, expecting to see them pass.

Open the UnitTest1.cs file and add using statements for the business logic and data-access layers that you created in the ContosoUniversity project:

using ContosoUniversity.BLL;
using ContosoUniversity.DAL;

Click here to view code as image

Replace the TestMethod1 method with the following methods:

private SchoolBL CreateSchoolBL()
{
var schoolRepository = new MockSchoolRepository();
var schoolBL = new SchoolBL(schoolRepository);
schoolBL.InsertDepartment(new Department() { Name = "First Department",
DepartmentID = 0, Administrator = 1, Person = new Instructor () { FirstMidName =
"Admin", LastName = "One" } });
schoolBL.InsertDepartment(new Department() { Name = "Second Department",
DepartmentID = 1, Administrator = 2, Person = new Instructor() { FirstMidName =
"Admin", LastName = "Two" } });
schoolBL.InsertDepartment(new Department() { Name = "Third Department",
DepartmentID = 2, Administrator = 3, Person = new Instructor() { FirstMidName =
"Admin", LastName = "Three" } });
return schoolBL;
}

[TestMethod]
[ExpectedException(typeof(DuplicateAdministratorException))]
public void AdministratorAssignmentRestrictionOnInsert()
{
var schoolBL = CreateSchoolBL();
schoolBL.InsertDepartment(new Department() { Name = "Fourth Department",
DepartmentID = 3, Administrator = 2, Person = new Instructor() { FirstMidName =
"Admin", LastName = "Two" } });
}

[TestMethod]
[ExpectedException(typeof(DuplicateAdministratorException))]
public void AdministratorAssignmentRestrictionOnUpdate()
{
var schoolBL = CreateSchoolBL();
var origDepartment = (from d in schoolBL.GetDepartments()
where d.Name == "Second Department"
select d).First();
var department = (from d in schoolBL.GetDepartments()
where d.Name == "Second Department"
select d).First();
department.Administrator = 1;
schoolBL.UpdateDepartment(department, origDepartment);
}

Click here to view code as image

The CreateSchoolBL method creates an instance of the repository class that you created for the unit test project, which it then passes to a new instance of the business-logic class. The method then uses the business-logic class to insert three departments that you can use in test methods.

The test methods verify that the business-logic class throws an exception if someone tries to insert a new department with the same administrator as an existing department, or if someone tries to update a department’s administrator by setting it to the ID of a person who is already the administrator of another department.

You haven’t created the exception class yet, so this code will not compile. To get it to compile, right-click DuplicateAdministratorException and select Generate, and then Class.

This creates a class in the test project which you can delete after you’ve created the exception class in the main project. and implemented the business logic.

Run the test project. As expected, the tests fail.

Adding Business Logic to Make a Test Pass

Next, you’ll implement the business logic that makes it impossible to set as the administrator of a department someone who is already administrator of another department. You’ll throw an exception from the business-logic layer, and then catch it in the presentation layer if a user edits a department and clicks Update after selecting someone who is already an administrator. (You could also remove instructors from the drop-down list who are already administrators before you render the page, but the purpose here is to work with the business-logic layer.)

Start by creating the exception class that you’ll throw when a user tries to make an instructor the administrator of more than one department. In the main project, create a new class file in the BLL folder, name it DuplicateAdministratorException.cs, and replace the existing code with the following code:

using System;

namespace ContosoUniversity.BLL
{
public class DuplicateAdministratorException : Exception
{
public DuplicateAdministratorException(string message)
: base(message)
{
}
}
}

Click here to view code as image

Now delete the temporary DuplicateAdministratorException.cs file that you created in the test project earlier in order to be able to compile.

In the main project, open the SchoolBL.cs file and add the following method that contains the validation logic. (The code refers to a method that you’ll create later.)

private void ValidateOneAdministratorAssignmentPerInstructor(Department
department)
{
if (department.Administrator != null)
{
var duplicateDepartment =
schoolRepository.GetDepartmentsByAdministrator(department.Administrator.GetValueOrDef
ault()).FirstOrDefault();
if (duplicateDepartment != null && duplicateDepartment.DepartmentID
!= department.DepartmentID)
{
throw new DuplicateAdministratorException(String.Format(
"Instructor {0} {1} is already administrator of the {2}
department.",
duplicateDepartment.Person.FirstMidName,
duplicateDepartment.Person.LastName,
duplicateDepartment.Name));
}
}
}

Click here to view code as image

You’ll call this method when you’re inserting or updating Department entities in order to check whether another department already has the same administrator.

The code calls a method to search the database for a Department entity that has the same Administrator property value as the entity being inserted or updated. If one is found, the code throws an exception. No validation check is required if the entity being inserted or updated has no Administrator value, and no exception is thrown if the method is called during an update and the Department entity found matches the Department entity being updated.

Call the new method from the Insert and Update methods:

public void InsertDepartment(Department department)
{
ValidateOneAdministratorAssignmentPerInstructor(department);
try
...

public void UpdateDepartment(Department department, Department
origDepartment)
{
ValidateOneAdministratorAssignmentPerInstructor(department);
try
...

Click here to view code as image

In ISchoolRepository.cs, add the following declaration for the new data-access method:

IEnumerable<Department> GetDepartmentsByAdministrator(Int32 administrator);

Click here to view code as image

In SchoolRepository.cs, add the following using statement:

using System.Data.Objects;

Click here to view code as image

In SchoolRepository.cs, add the following new data-access method:

public IEnumerable<Department> GetDepartmentsByAdministrator(Int32
administrator)
{
return new ObjectQuery<Department>("SELECT VALUE d FROM Departments as
d", context, MergeOption.NoTracking).Include("Person").Where(d => d.Administrator ==
administrator).ToList();
}

Click here to view code as image

This code retrieves Department entities that have a specified administrator. Only one department should be found (if any). However, because no constraint is built into the database, the return type is a collection in case multiple departments are found.

By default, when the object context retrieves entities from the database, it keeps track of them in its object state manager. The MergeOption.NoTracking parameter specifies that this tracking will not be done for this query. This is necessary because the query might return the exact entity that you’re trying to update, and then you would not be able to attach that entity. For example, if you edit the History department in the Departments.aspx page and leave the administrator unchanged, this query will return the History department. If NoTracking is not set, the object context would already have the History department entity in its object state manager. Then when you attach the History department entity that’s re-created from view state, the object context would throw an exception that says "An object with the same key already exists in the ObjectStateManager. The ObjectStateManager cannot track multiple objects with the same key".

(As an alternative to specifying MergeOption.NoTracking, you could create a new object context just for this query. Because the new object context would have its own object state manager, there would be no conflict when you call the Attach method. The new object context would share metadata and database connection with the original object context, so the performance penalty of this alternate approach would be minimal. The approach shown here, however, introduces the NoTracking option, which you’ll find useful in other contexts. The NoTracking option is discussed further in a later tutorial in this series.)

In the test project, add the new data-access method to MockSchoolRepository.cs:

public IEnumerable<Department> GetDepartmentsByAdministrator(Int32
administrator)
{
return (from d in departments
where d.Administrator == administrator
select d);
}

Click here to view code as image

This code uses LINQ to perform the same data selection that the ContosoUniversity project repository uses LINQ to Entities for.

Run the test project again. This time the tests pass.

Handling ObjectDataSource Exceptions

In the ContosoUniversity project, run the Departments.aspx page and try to change the administrator for a department to someone who is already administrator for another department. (Remember that you can only edit departments that you added during this tutorial, because the database comes preloaded with invalid data.) You get the following server error page:

You don’t want users to see this kind of error page, so you need to add error-handling code. Open Departments.aspx and specify a handler for the OnUpdated event of the DepartmentsObjectDataSource. The ObjectDataSource opening tag now resembles the following example.

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.BLL.SchoolBL"
DataObjectTypeName="ContosoUniversity.DAL.Department"
SelectMethod="GetDepartments"
DeleteMethod="DeleteDepartment"
UpdateMethod="UpdateDepartment"
ConflictDetection="CompareAllValues"
OldValuesParameterFormatString="orig{0}"
OnUpdated="DepartmentsObjectDataSource_Updated" >

Click here to view code as image

In Departments.aspx.cs, add the following using statement:

using ContosoUniversity.BLL;

Click here to view code as image

Add the following handler for the Updated event:

protected void DepartmentsObjectDataSource_Updated(object sender,
ObjectDataSourceStatusEventArgs e)
{
if (e.Exception != null)
{
if (e.Exception.InnerException is DuplicateAdministratorException)
{
var duplicateAdministratorValidator = new CustomValidator();
duplicateAdministratorValidator.IsValid = false;
duplicateAdministratorValidator.ErrorMessage = "Update failed: "
+ e.Exception.InnerException.Message;
Page.Validators.Add(duplicateAdministratorValidator);
e.ExceptionHandled = true;
}
}
}

Click here to view code as image

If the ObjectDataSource control catches an exception when it tries to perform the update, it passes the exception in the event argument (e) to this handler. The code in the handler checks to see if the exception is the duplicate administrator exception. If it is, the code creates a validator control that contains an error message for the ValidationSummary control to display.

Run the page and attempt to make someone the administrator of two departments again. This time the ValidationSummary control displays an error message.

Make similar changes to the DepartmentsAdd.aspx page. In DepartmentsAdd.aspx, specify a handler for the OnInserted event of the DepartmentsObjectDataSource. The resulting markup will resemble the following example.

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.BLL.SchoolBL"
DataObjectTypeName="ContosoUniversity.DAL.Department"
InsertMethod="InsertDepartment"
OnInserted="DepartmentsObjectDataSource_Inserted">

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In DepartmentsAdd.aspx.cs, add the same using statement:

using ContosoUniversity.BLL;

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Add the following event handler:

protected void DepartmentsObjectDataSource_Inserted(object sender,
ObjectDataSourceStatusEventArgs e)
{
if (e.Exception != null)
{
if (e.Exception.InnerException is DuplicateAdministratorException)
{
var duplicateAdministratorValidator = new CustomValidator();
duplicateAdministratorValidator.IsValid = false;
duplicateAdministratorValidator.ErrorMessage = "Insert failed: "
+ e.Exception.InnerException.Message;
Page.Validators.Add(duplicateAdministratorValidator);
e.ExceptionHandled = true;
}
}
}

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You can now test the DepartmentsAdd.aspx.cs page to verify that it also correctly handles attempts to make one person the administrator of more than one department.

This completes the introduction to implementing the repository pattern for using the ObjectDataSource control with the Entity Framework. For more information about the repository pattern and testability, see the MSDN whitepaper Testability and Entity Framework 4.0.

In the following tutorial you’ll see how to add sorting and filtering functionality to the application.

Sorting and Filtering

In the previous tutorial you implemented the repository pattern in an n-tier web application that uses the Entity Framework and the ObjectDataSource control. This tutorial shows how to do sorting and filtering and handle master-detail scenarios. You’ll add the following enhancements to the Departments.aspx page:

• A text box to allow users to select departments by name.

• A list of courses for each department that’s shown in the grid.

• The ability to sort by clicking column headings.

Adding the Ability to Sort GridView Columns

Open the Departments.aspx page and add a SortParameterName="sortExpression" attribute to the ObjectDataSource control named DepartmentsObjectDataSource. (Later you’ll create a GetDepartments method that takes a parameter named sortExpression.) The markup for the opening tag of the control now resembles the following example.

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.BLL.SchoolBL"
DataObjectTypeName="ContosoUniversity.DAL.Department"
SelectMethod="GetDepartments" DeleteMethod="DeleteDepartment"
UpdateMethod="UpdateDepartment"
ConflictDetection="CompareAllValues" OldValuesParameterFormatString="orig{0}"
OnUpdated="DepartmentsObjectDataSource_Updated"
SortParameterName="sortExpression" >

Click here to view code as image

Add the AllowSorting="true" attribute to the opening tag of the GridView control. The markup for the opening tag of the control now resembles the following example.

<asp:GridView ID="DepartmentsGridView" runat="server" AutoGenerateColumns="False"
DataSourceID="DepartmentsObjectDataSource" DataKeyNames="DepartmentID"
OnRowUpdating="DepartmentsGridView_RowUpdating"
AllowSorting="true" >

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In Departments.aspx.cs, set the default sort order by calling the GridView control’s Sort method from the Page_Load method:

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
DepartmentsGridView.Sort("Name", SortDirection.Ascending);
}
}

Click here to view code as image

You can add code that sorts or filters in either the business logic class or the repository class. If you do it in the business logic class, the sorting or filtering work will be done after the data is retrieved from the database, because the business logic class is working with an IEnumerable object returned by the repository. If you add sorting and filtering code in the repository class and you do it before a LINQ expression or object query has been converted to an IEnumerable object, your commands will be passed through to the database for processing, which is typically more efficient. In this tutorial you’ll implement sorting and filtering in a way that causes the processing to be done by the database — that is, in the repository.

To add sorting capability, you must add a new method to the repository interface and repository classes as well as to the business logic class. In the ISchoolRepository.cs file, add a new GetDepartments method that takes a sortExpression parameter that will be used to sort the list of departments that’s returned:

IEnumerable<Department> GetDepartments(string sortExpression);

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The sortExpression parameter will specify the column to sort on and the sort direction.

Add code for the new method to the SchoolRepository.cs file:

public IEnumerable<Department> GetDepartments(string sortExpression)
{
if (String.IsNullOrWhiteSpace(sortExpression))
{
sortExpression = "Name";
}
return context.Departments.Include("Person").OrderBy("it." +
sortExpression).ToList();
}

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Change the existing parameterless GetDepartments method to call the new method:

public IEnumerable<Department> GetDepartments()
{
return GetDepartments("");
}

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In the test project, add the following new method to MockSchoolRepository.cs:

public IEnumerable<Department> GetDepartments(string sortExpression)
{
return departments;
}

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If you were going to create any unit tests that depended on this method returning a sorted list, you would need to sort the list before returning it. You won’t be creating tests like that in this tutorial, so the method can just return the unsorted list of departments.

In the SchoolBL.cs file, add the following new method to the business logic class:

public IEnumerable<Department> GetDepartments(string sortExpression)
{
return schoolRepository.GetDepartments(sortExpression);
}

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This code passes the sort parameter to the repository method.

Run the Departments.aspx page.

You can now click any column heading to sort by that column. If the column is already sorted, clicking the heading reverses the sort direction.

Adding a Search Box

In this section you’ll add a search text box, link it to the ObjectDataSource control using a control parameter, and add a method to the business logic class to support filtering.

Open the Departments.aspx page and add the following markup between the heading and the first ObjectDataSource control:

Enter any part of the name or leave the box blank to see all names:
<asp:TextBox ID="SearchTextBox" runat="server" AutoPostBack="true"></asp:TextBox>
<asp:Button ID="SearchButton" runat="server" Text="Search" />

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In the ObjectDataSource control named DepartmentsObjectDataSource, do the following:

• Add a SelectParameters element for a parameter named nameSearchString that gets the value entered in the SearchTextBox control.

• Change the SelectMethod attribute value to GetDepartmentsByName. (You’ll create this method later.)

The markup for the ObjectDataSource control now resembles the following example:

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.BLL.SchoolBL"
SelectMethod="GetDepartmentsByName" DeleteMethod="DeleteDepartment"
UpdateMethod="UpdateDepartment"
DataObjectTypeName="ContosoUniversity.DAL.Department"
ConflictDetection="CompareAllValues"
SortParameterName="sortExpression" OldValuesParameterFormatString="orig{0}"
OnUpdated="DepartmentsObjectDataSource_Updated">
<SelectParameters>
<asp:ControlParameter ControlID="SearchTextBox" Name="nameSearchString"
PropertyName="Text"
Type="String" />
</SelectParameters>
</asp:ObjectDataSource>

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In ISchoolRepository.cs, add a GetDepartmentsByName method that takes both sortExpression and nameSearchString parameters:

IEnumerable<Department> GetDepartmentsByName(string sortExpression, string
nameSearchString);

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In SchoolRepository.cs, add the following new method:

public IEnumerable<Department> GetDepartmentsByName(string sortExpression,
string nameSearchString)
{
if (String.IsNullOrWhiteSpace(sortExpression))
{
sortExpression = "Name";
}
if (String.IsNullOrWhiteSpace(nameSearchString))
{
nameSearchString = "";
}
return context.Departments.Include("Person").OrderBy("it." +
sortExpression).Where(d => d.Name.Contains(nameSearchString)).ToList();
}

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This code uses a Where method to select items that contain the search string. If the search string is empty, all records will be selected. Note that when you specify method calls together in one statement like this (Include, then OrderBy, then Where), the Where method must always be last.

Change the existing GetDepartments method that takes a sortExpression parameter to call the new method:

public IEnumerable<Department> GetDepartments(string sortExpression)
{
return GetDepartmentsByName(sortExpression, "");
}

Click here to view code as image

In MockSchoolRepository.cs in the test project, add the following new method:

public IEnumerable<Department> GetDepartmentsByName(string sortExpression,
string nameSearchString)
{
return departments;
}

Click here to view code as image

In SchoolBL.cs, add the following new method:

public IEnumerable<Department> GetDepartmentsByName(string sortExpression,
string nameSearchString)
{
return schoolRepository.GetDepartmentsByName(sortExpression,
nameSearchString);
}

Click here to view code as image

Run the Departments.aspx page and enter a search string to make sure that the selection logic works. Leave the text box empty and try a search to make sure that all records are returned.

Adding a Details Column for Each Grid Row

Next, you want to see all of the courses for each department displayed in the right-hand cell of the grid. To do this, you’ll use a nested GridView control and databind it to data from the Courses navigation property of the Department entity.

Open Departments.aspx and in the markup for the GridView control, specify a handler for the RowDataBound event. The markup for the opening tag of the control now resembles the following example.

<asp:GridView ID="DepartmentsGridView" runat="server" AutoGenerateColumns="False"
DataSourceID="DepartmentsObjectDataSource" DataKeyNames="DepartmentID"
OnRowUpdating="DepartmentsGridView_RowUpdating"
OnRowDataBound="DepartmentsGridView_RowDataBound"
AllowSorting="True" >

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Add a new TemplateField element after the Administrator template field:

<asp:TemplateField HeaderText="Courses">
<ItemTemplate>
<asp:GridView ID="CoursesGridView" runat="server"
AutoGenerateColumns="False">
<Columns>
<asp:BoundField DataField="CourseID" HeaderText="ID" />
<asp:BoundField DataField="Title" HeaderText="Title" />
</Columns>
</asp:GridView>
</ItemTemplate>
</asp:TemplateField>

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This markup creates a nested GridView control that shows the course number and title of a list of courses. It does not specify a data source because you’ll databind it in code in the RowDataBound handler.

Open Departments.aspx.cs and add the following handler for the RowDataBound event:

protected void DepartmentsGridView_RowDataBound(object sender,
GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
var department = e.Row.DataItem as Department;
var coursesGridView = (GridView)e.Row.FindControl("CoursesGridView");
coursesGridView.DataSource = department.Courses.ToList();
coursesGridView.DataBind();
}
}

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This code gets the Department entity from the event arguments, converts the Courses navigation property to a List collection, and databinds the nested GridView to the collection.

Open the SchoolRepository.cs file and specify eager loading for the Courses navigation property by calling the Include method in the object query that you create in the GetDepartmentsByName method. The return statement in the GetDepartmentsByName method now resembles the following example.

return context.Departments.Include("Person").Include("Courses").
OrderBy("it." + sortExpression).Where(d =>
d.Name.Contains(nameSearchString)).ToList();

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Run the page. In addition to the sorting and filtering capability that you added earlier, the GridView control now shows nested course details for each department.

This completes the introduction to sorting, filtering, and master-detail scenarios. In the next tutorial, you’ll see how to handle concurrency.

Handling Concurrency

By Tom Dykstra|January 26, 2011

This tutorial series builds on the Contoso University web application that is created by the Getting Started with the Entity Framework tutorial series. If you didn’t complete the earlier tutorials, as a starting point for this tutorial you can download the application that you would have created. You can also download the application that is created by the complete tutorial series. If you have questions about the tutorials, you can post them to the ASP.NET Entity Framework forum.

In the previous tutorial you learned how to sort and filter data using the ObjectDataSource control and the Entity Framework. This tutorial shows options for handling concurrency in an ASP.NET web application that uses the Entity Framework. You will create a new web page that’s dedicated to updating instructor office assignments. You’ll handle concurrency issues in that page and in the Departments page that you created earlier.

Concurrency Conflicts

A concurrency conflict occurs when one user edits a record and another user edits the same record before the first user’s change is written to the database. If you don’t set up the Entity Framework to detect such conflicts, whoever updates the database last overwrites the other user’s changes. In many applications, this risk is acceptable, and you don’t have to configure the application to handle possible concurrency conflicts. (If there are few users, or few updates, or if isn’t really critical if some changes are overwritten, the cost of programming for concurrency might outweigh the benefit.) If you don’t need to worry about concurrency conflicts, you can skip this tutorial; the remaining two tutorials in the series don’t depend on anything you build in this one.

Pessimistic Concurrency (Locking)

If your application does need to prevent accidental data loss in concurrency scenarios, one way to do that is to use database locks. This is called pessimistic concurrency. For example, before you read a row from a database, you request a lock for read-only or for update access. If you lock a row for update access, no other users are allowed to lock the row either for read-only or update access, because they would get a copy of data that’s in the process of being changed. If you lock a row for read-only access, others can also lock it for read-only access but not for update.

Managing locks has some disadvantages. It can be complex to program. It requires significant database management resources, and it can cause performance problems as the number of users of an application increases (that is, it doesn’t scale well). For these reasons, not all database management systems support pessimistic concurrency. The Entity Framework provides no built-in support for it, and this tutorial doesn’t show you how to implement it.

Optimistic Concurrency

The alternative to pessimistic concurrency is optimistic concurrency. Optimistic concurrency means allowing concurrency conflicts to happen, and then reacting appropriately if they do. For example, John runs the Department.aspx page, clicks the Edit link for the History department, and reduces the Budget amount from $1,000,000.00 to $125,000.00. (John administers a competing department and wants to free up money for his own department.)

Before John clicks Update, Jane runs the same page, clicks the Edit link for the History department, and then changes the Start Date field from 1/10/2011 to 1/1/1999. (Jane administers the History department and wants to give it more seniority.)

John clicks Update first, then Jane clicks Update. Jane’s browser now lists the Budget amount as $1,000,000.00, but this is incorrect because the amount has been changed by John to $125,000.00.

Some of the actions you can take in this scenario include the following:

• You can keep track of which property a user has modified and update only the corresponding columns in the database. In the example scenario, no data would be lost, because different properties were updated by the two users. The next time someone browses the History department, they will see 1/1/999 and $125,000.00.

This is the default behavior in the Entity Framework, and it can substantially reduce the number of conflicts that could result in data loss. However, this behavior doesn’t avoid data loss if competing changes are made to the same property of an entity. In addition, this behavior isn’t always possible; when you map stored procedures to an entity type, all of an entity’s properties are updated when any changes to the entity are made in the database.

• You can let Jane’s change overwrite John’s change. After Jane clicks Update, the Budget amount goes back to $1,000,000.00. This is called a Client Wins or Last in Wins scenario. (The client’s values take precedence over what’s in the data store.)

• You can prevent Jane’s change from being updated in the database. Typically, you would display an error message, show her the current state of the data, and allow her to reenter her changes if she still wants to make them. You could further automate the process by saving her input and giving her an opportunity to reapply it without having to reenter it. This is called a Store Wins scenario. (The data-store values take precedence over the values submitted by the client.)

Detecting Concurrency Conflicts

In the Entity Framework, you can resolve conflicts by handling OptimisticConcurrencyException exceptions that the Entity Framework throws. In order to know when to throw these exceptions, the Entity Framework must be able to detect conflicts. Therefore, you must configure the database and the data model appropriately. Some options for enabling conflict detection include the following:

• In the database, include a table column that can be used to determine when a row has been changed. You can then configure the Entity Framework to include that column in the Where clause of SQL Update or Delete commands.

That’s the purpose of the Timestamp column in the OfficeAssignment table.

The data type of the Timestamp column is also called Timestamp. However, the column doesn’t actually contain a date or time value. Instead, the value is a sequential number that’s incremented each time the row is updated. In an Update or Delete command, the Where clause includes the original Timestamp value. If the row being updated has been changed by another user, the value in Timestamp is different than the original value, so the Where clause returns no row to update. When the Entity Framework finds that no rows have been updated by the current Update or Delete command (that is, when the number of affected rows is zero), it interprets that as a concurrency conflict.

• Configure the Entity Framework to include the original values of every column in the table in the Where clause of Update and Delete commands.

As in the first option, if anything in the row has changed since the row was first read, the Where clause won’t return a row to update, which the Entity Framework interprets as a concurrency conflict. This method is as effective as using a Timestamp field, but can be inefficient. For database tables that have many columns, it can result in very large Where clauses, and in a web application it can require that you maintain large amounts of state. Maintaining large amounts of state can affect application performance because it either requires server resources (for example, session state) or must be included in the web page itself (for example, view state).

In this tutorial you will add error handling for optimistic concurrency conflicts for an entity that doesn’t have a tracking property (the Department entity) and for an entity that does have a tracking property (the OfficeAssignment entity).

Handling Optimistic Concurrency Without a Tracking Property

To implement optimistic concurrency for the Department entity, which doesn’t have a tracking (Timestamp) property, you will complete the following tasks:

• Change the data model to enable concurrency tracking for Department entities.

• In the SchoolRepository class, handle concurrency exceptions in the SaveChanges method.

• In the Departments.aspx page, handle concurrency exceptions by displaying a message to the user warning that the attempted changes were unsuccessful. The user can then see the current values and retry the changes if they are still needed.

Enabling Concurrency Tracking in the Data Model

In Visual Studio, open the Contoso University web application that you were working with in the previous tutorial in this series.

Open SchoolModel.edmx, and in the data model designer, right-click the Name property in the Department entity and then click Properties. In the Properties window, change the ConcurrencyMode property to Fixed.

Do the same for the other non-primary-key scalar properties (Budget, StartDate, and Administrator.) (You can’t do this for navigation properties.) This specifies that whenever the Entity Framework generates a Update or Delete SQL command to update the Department entity in the database, these columns (with original values) must be included in the Where clause. If no row is found when the Update or Delete command executes, the Entity Framework will throw an optimistic-concurrency exception.

Save and close the data model.

Handling Concurrency Exceptions in the DAL

Open SchoolRepository.cs and add the following using statement for the System.Data namespace:

using System.Data;

Click here to view code as image

Add the following new SaveChanges method, which handles optimistic concurrency exceptions:

public void SaveChanges()
{
try
{
context.SaveChanges();
}
catch (OptimisticConcurrencyException ocex)
{
context.Refresh(RefreshMode.StoreWins, ocex.StateEntries[0].Entity);
throw ocex;
}
}

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If a concurrency error occurs when this method is called, the property values of the entity in memory are replaced with the values currently in the database. The concurrency exception is rethrown so that the web page can handle it.

In the DeleteDepartment and UpdateDepartment methods, replace the existing call to context.SaveChanges() with a call to SaveChanges() in order to invoke the new method.

Handling Concurrency Exceptions in the Presentation Layer

Open Departments.aspx and add an OnDeleted="DepartmentsObjectDataSource_Deleted" attribute to the DepartmentsObjectDataSource control. The opening tag for the control will now resemble the following example.

<asp:ObjectDataSource ID="DepartmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.BLL.SchoolBL"
DataObjectTypeName="ContosoUniversity.DAL.Department"
SelectMethod="GetDepartmentsByName" DeleteMethod="DeleteDepartment"
UpdateMethod="UpdateDepartment"
ConflictDetection="CompareAllValues" OldValuesParameterFormatString="orig{0}"
OnUpdated="DepartmentsObjectDataSource_Updated"
SortParameterName="sortExpression"
OnDeleted="DepartmentsObjectDataSource_Deleted" >

Click here to view code as image

In the DepartmentsGridView control, specify all of the table columns in the DataKeyNames attribute, as shown in the following example. Note that this will create very large view state fields, which is one reason why using a tracking field is generally the preferred way to track concurrency conflicts.

<asp:GridView ID="DepartmentsGridView" runat="server" AutoGenerateColumns="False"
DataSourceID="DepartmentsObjectDataSource"
DataKeyNames="DepartmentID,Name,Budget,StartDate,Administrator"
OnRowUpdating="DepartmentsGridView_RowUpdating"
OnRowDataBound="DepartmentsGridView_RowDataBound"
AllowSorting="True" >

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Open Departments.aspx.cs and add the following using statement for the System.Data namespace:

using System.Data;

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Add the following new method, which you will call from the data source control’s Updated and Deleted event handlers for handling concurrency exceptions:

private void
CheckForOptimisticConcurrencyException(ObjectDataSourceStatusEventArgs e, string
function)
{
if (e.Exception.InnerException is OptimisticConcurrencyException)
{
var concurrencyExceptionValidator = new CustomValidator();
concurrencyExceptionValidator.IsValid = false;
concurrencyExceptionValidator.ErrorMessage =
"The record you attempted to edit or delete was modified by
another " +
"user after you got the original value. The edit or delete
operation was canceled " +
"and the other user's values have been displayed so you can " +
"determine whether you still want to edit or delete this
record.";
Page.Validators.Add(concurrencyExceptionValidator);
e.ExceptionHandled = true;
}
}

Click here to view code as image

This code checks the exception type, and if it’s a concurrency exception, the code dynamically creates a CustomValidator control that in turn displays a message in the ValidationSummary control.

Call the new method from the Updated event handler that you added earlier. In addition, create a new Deleted event handler that calls the same method (but doesn’t do anything else):

protected void DepartmentsObjectDataSource_Updated(object sender,
ObjectDataSourceStatusEventArgs e)
{
if (e.Exception != null)
{
CheckForOptimisticConcurrencyException(e, "update");
// ...
}
}

protected void DepartmentsObjectDataSource_Deleted(object sender,
ObjectDataSourceStatusEventArgs e)
{
if (e.Exception != null)
{
CheckForOptimisticConcurrencyException(e, "delete");
}
}

Click here to view code as image

Testing Optimistic Concurrency in the Departments Page

Run the Departments.aspx page.

Click Edit in a row and change the value in the Budget column. (Remember that you can only edit records that you’ve created for this tutorial, because the existing School database records contain some invalid data. The record for the Economics department is a safe one to experiment with.)

Open a new browser window and run the page again (copy the URL from the first browser window’s address box to the second browser window).

Click Edit in the same row you edited earlier and change the Budget value to something different.

In the second browser window, click Update. The Budget amount is successfully changed to this new value.

In the first browser window, click Update. The update fails. The Budget amount is redisplayed using the value you set in the second browser window, and you see an error message.

Handling Optimistic Concurrency Using a Tracking Property

To handle optimistic concurrency for an entity that has a tracking property, you will complete the following tasks:

• Add stored procedures to the data model to manage OfficeAssignment entities. (Tracking properties and stored procedures don’t have to be used together; they’re just grouped together here for illustration.)

• Add CRUD methods to the DAL and the BLL for OfficeAssignment entities, including code to handle optimistic concurrency exceptions in the DAL.

• Create an office-assignments web page.

• Test optimistic concurrency in the new web page.

Adding OfficeAssignment Stored Procedures to the Data Model

Open the SchoolModel.edmx file in the model designer, right-click the design surface, and click Update Model from Database. In the Add tab of the Choose Your Database Objects dialog box, expand Stored Procedures and select the three OfficeAssignment stored procedures (see the following screenshot), and then click Finish. (These stored procedures were already in the database when you downloaded or created it using a script.)

Right-click the OfficeAssignment entity and select Stored Procedure Mapping.

Set the Insert, Update, and Delete functions to use their corresponding stored procedures. For the OrigTimestamp parameter of the Update function, set the Property to Timestamp and select the Use Original Value option.

When the Entity Framework calls the UpdateOfficeAssignment stored procedure, it will pass the original value of the Timestamp column in the OrigTimestamp parameter. The stored procedure uses this parameter in its Where clause:

ALTER PROCEDURE [dbo].[UpdateOfficeAssignment]
@InstructorID int,
@Location nvarchar(50),
@OrigTimestamp timestamp
AS
UPDATE OfficeAssignment SET Location=@Location
WHERE InstructorID=@InstructorID AND [Timestamp]=@OrigTimestamp;
IF @@ROWCOUNT > 0
BEGIN
SELECT [Timestamp] FROM OfficeAssignment
WHERE InstructorID=@InstructorID;
END

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The stored procedure also selects the new value of the Timestamp column after the update so that the Entity Framework can keep the OfficeAssignment entity that’s in memory in sync with the corresponding database row.

(Note that the stored procedure for deleting an office assignment doesn’t have an OrigTimestamp parameter. Because of this, the Entity Framework can’t verify that an entity is unchanged before deleting it.)

Save and close the data model.

Adding OfficeAssignment Methods to the DAL

Open ISchoolRepository.cs and add the following CRUD methods for the OfficeAssignment entity set:

IEnumerable<OfficeAssignment> GetOfficeAssignments(string sortExpression);
void InsertOfficeAssignment(OfficeAssignment OfficeAssignment);
void DeleteOfficeAssignment(OfficeAssignment OfficeAssignment);
void UpdateOfficeAssignment(OfficeAssignment OfficeAssignment,
OfficeAssignment origOfficeAssignment);

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Add the following new methods to SchoolRepository.cs. In the UpdateOfficeAssignment method, you call the local SaveChanges method instead of context.SaveChanges.

public IEnumerable<OfficeAssignment> GetOfficeAssignments(string
sortExpression)
{
return new ObjectQuery<OfficeAssignment>("SELECT VALUE o FROM
OfficeAssignments AS o", context).Include("Person").OrderBy("it." +
sortExpression).ToList();
}

public void InsertOfficeAssignment(OfficeAssignment officeAssignment)
{
context.OfficeAssignments.AddObject(officeAssignment);
context.SaveChanges();
}

public void DeleteOfficeAssignment(OfficeAssignment officeAssignment)
{
context.OfficeAssignments.Attach(officeAssignment);
context.OfficeAssignments.DeleteObject(officeAssignment);
context.SaveChanges();
}

public void UpdateOfficeAssignment(OfficeAssignment officeAssignment,
OfficeAssignment origOfficeAssignment)
{
context.OfficeAssignments.Attach(origOfficeAssignment);
context.ApplyCurrentValues("OfficeAssignments", officeAssignment);
SaveChanges();
}

Click here to view code as image

In the test project, open MockSchoolRepository.cs and add the following OfficeAssignment collection and CRUD methods to it. (The mock repository must implement the repository interface, or the solution won’t compile.)

List<OfficeAssignment> officeAssignments = new List<OfficeAssignment>();

public IEnumerable<OfficeAssignment> GetOfficeAssignments(string
sortExpression)
{
return officeAssignments;
}

public void InsertOfficeAssignment(OfficeAssignment officeAssignment)
{
officeAssignments.Add(officeAssignment);
}

public void DeleteOfficeAssignment(OfficeAssignment officeAssignment)
{
officeAssignments.Remove(officeAssignment);
}

public void UpdateOfficeAssignment(OfficeAssignment officeAssignment,
OfficeAssignment origOfficeAssignment)
{
officeAssignments.Remove(origOfficeAssignment);
officeAssignments.Add(officeAssignment);
}

Click here to view code as image

Adding OfficeAssignment Methods to the BLL

In the main project, open SchoolBL.cs and add the following CRUD methods for the OfficeAssignment entity set to it:

public IEnumerable<OfficeAssignment> GetOfficeAssignments(string
sortExpression)
{
if (string.IsNullOrEmpty(sortExpression)) sortExpression =
"Person.LastName";
return schoolRepository.GetOfficeAssignments(sortExpression);
}

public void InsertOfficeAssignment(OfficeAssignment officeAssignment)
{
try
{
schoolRepository.InsertOfficeAssignment(officeAssignment);
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

public void DeleteOfficeAssignment(OfficeAssignment officeAssignment)
{
try
{
schoolRepository.DeleteOfficeAssignment(officeAssignment);
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

public void UpdateOfficeAssignment(OfficeAssignment officeAssignment,
OfficeAssignment origOfficeAssignment)
{
try
{
schoolRepository.UpdateOfficeAssignment(officeAssignment,
origOfficeAssignment);
}
catch (Exception ex)
{
//Include catch blocks for specific exceptions first,
//and handle or log the error as appropriate in each.
//Include a generic catch block like this one last.
throw ex;
}
}

Click here to view code as image

Creating an OfficeAssignments Web Page

Create a new web page that uses the Site.Master master page and name it OfficeAssignments.aspx. Add the following markup to the Content control named Content2:

<h2>Office Assignments</h2>
<asp:ObjectDataSource ID="OfficeAssignmentsObjectDataSource" runat="server"
TypeName="ContosoUniversity.BLL.SchoolBL"
DataObjectTypeName="ContosoUniversity.DAL.OfficeAssignment"
SelectMethod="GetOfficeAssignments"
DeleteMethod="DeleteOfficeAssignment" UpdateMethod="UpdateOfficeAssignment"
ConflictDetection="CompareAllValues"
OldValuesParameterFormatString="orig{0}"
SortParameterName="sortExpression"
OnUpdated="OfficeAssignmentsObjectDataSource_Updated">
</asp:ObjectDataSource>
<asp:ValidationSummary ID="OfficeAssignmentsValidationSummary" runat="server"
ShowSummary="true"
DisplayMode="BulletList" Style="color: Red; width: 40em;" />
<asp:GridView ID="OfficeAssignmentsGridView" runat="server"
AutoGenerateColumns="False"
DataSourceID="OfficeAssignmentsObjectDataSource"
DataKeyNames="InstructorID,Timestamp"
AllowSorting="True">
<Columns>
<asp:CommandField ShowEditButton="True" ShowDeleteButton="True"
ItemStyle-VerticalAlign="Top">
<ItemStyle VerticalAlign="Top"></ItemStyle>
</asp:CommandField>
<asp:TemplateField HeaderText="Instructor"
SortExpression="Person.LastName">
<ItemTemplate>
<asp:Label ID="InstructorLastNameLabel" runat="server" Text='<%#
Eval("Person.LastName") %>'></asp:Label>,
<asp:Label ID="InstructorFirstNameLabel" runat="server" Text='<%#
Eval("Person.FirstMidName") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
<asp:DynamicField DataField="Location" HeaderText="Location"
SortExpression="Location"/>
</Columns>
<SelectedRowStyle BackColor="LightGray"></SelectedRowStyle>
</asp:GridView>

Click here to view code as image

Notice that in the DataKeyNames attribute, the markup specifies the Timestamp property as well as the record key (InstructorID). Specifying properties in the DataKeyNames attribute causes the control to save them in control state (which is similar to view state) so that the original values are available during postback processing.

If you didn’t save the Timestamp value, the Entity Framework would not have it for the Where clause of the SQL Update command. Consequently nothing would be found to update. As a result, the Entity Framework would throw an optimistic concurrency exception every time an OfficeAssignment entity is updated.

Open OfficeAssignments.aspx.cs and add the following using statement for the data access layer:

using ContosoUniversity.DAL;

Click here to view code as image

Add the following Page_Init method, which enables Dynamic Data functionality. Also add the following handler for the ObjectDataSource control’s Updated event in order to check for concurrency errors:

protected void Page_Init(object sender, EventArgs e)
{
OfficeAssignmentsGridView.EnableDynamicData(typeof(OfficeAssignment));
}

protected void OfficeAssignmentsObjectDataSource_Updated(object sender,
ObjectDataSourceStatusEventArgs e)
{
if (e.Exception != null)
{
var concurrencyExceptionValidator = new CustomValidator();
concurrencyExceptionValidator.IsValid = false;
concurrencyExceptionValidator.ErrorMessage = "The record you
attempted to " +
"update has been modified by another user since you last visited
this page. " +
"Your update was canceled to allow you to review the other user's
" +
"changes and determine if you still want to update this record.";
Page.Validators.Add(concurrencyExceptionValidator);
e.ExceptionHandled = true;
}
}

Click here to view code as image

Testing Optimistic Concurrency in the OfficeAssignments Page

Run the OfficeAssignments.aspx page.

Click Edit in a row and change the value in the Location column.

Open a new browser window and run the page again (copy the URL from the first browser window to the second browser window).

Click Edit in the same row you edited earlier and change the Location value to something different.

In the second browser window, click Update.

Switch to the first browser window and click Update.

You see an error message and the Location value has been updated to show the value you changed it to in the second browser window.

Handling Concurrency with the EntityDataSource Control

The EntityDataSource control includes built-in logic that recognizes the concurrency settings in the data model and handles update and delete operations accordingly. However, as with all exceptions, you must handle OptimisticConcurrencyException exceptions yourself in order to provide a user-friendly error message.

Next, you will configure the Courses.aspx page (which uses an EntityDataSource control) to allow update and delete operations and to display an error message if a concurrency conflict occurs. The Course entity doesn’t have a concurrency tracking column, so you will use the same method that you did with the Department entity: track the values of all non-key properties.

Open the SchoolModel.edmx file. For the non-key properties of the Course entity (Title, Credits, and DepartmentID), set the Concurrency Mode property to Fixed. Then save and close the data model.

Open the Courses.aspx page and make the following changes:

• In the CoursesEntityDataSource control, add EnableUpdate="true" and EnableDelete="true" attributes. The opening tag for that control now resembles the following example:

<asp:EntityDataSource ID="CoursesEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="false"
AutoGenerateWhereClause="True" EntitySetName="Courses"
EnableUpdate="true" EnableDelete="true">

Click here to view code as image

• In the CoursesGridView control, change the DataKeyNames attribute value to "CourseID,Title,Credits,DepartmentID". Then add a CommandField element to the Columns element that shows Edit and Delete buttons (<asp:CommandField ShowEditButton="True" ShowDeleteButton="True" />). The GridView control now resembles the following example:

<asp:GridView ID="CoursesGridView" runat="server" AutoGenerateColumns="False"
DataKeyNames="CourseID,Title,Credits,DepartmentID"
DataSourceID="CoursesEntityDataSource" >
<Columns>
<asp:CommandField ShowEditButton="True" ShowDeleteButton="True" />
<asp:BoundField DataField="CourseID" HeaderText="CourseID"
ReadOnly="True" SortExpression="CourseID" />
<asp:BoundField DataField="Title" HeaderText="Title"
SortExpression="Title" />
<asp:BoundField DataField="Credits" HeaderText="Credits"
SortExpression="Credits" />
</Columns>
</asp:GridView>

Click here to view code as image

Run the page and create a conflict situation as you did before in the Departments page. Run the page in two browser windows, click Edit in the same line in each window, and make a different change in each one. Click Update in one window and then click Update in the other window. When you click Update the second time, you see the error page that results from an unhandled concurrency exception.

You handle this error in a manner very similar to how you handled it for the ObjectDataSource control. Open the Courses.aspx page, and in the CoursesEntityDataSource control, specify handlers for the Deleted and Updated events. The opening tag of the control now resembles the following example:

<asp:EntityDataSource ID="CoursesEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.SchoolEntities"
EnableFlattening="false"
AutoGenerateWhereClause="true" EntitySetName="Courses"
EnableUpdate="true" EnableDelete="true"
OnDeleted="CoursesEntityDataSource_Deleted"
OnUpdated="CoursesEntityDataSource_Updated">

Click here to view code as image

Before the CoursesGridView control, add the following ValidationSummary control:

<asp:ValidationSummary ID="CoursesValidationSummary" runat="server"
ShowSummary="true" DisplayMode="BulletList" />

Click here to view code as image

In Courses.aspx.cs, add a using statement for the System.Data namespace, add a method that checks for concurrency exceptions, and add handlers for the EntityDataSource control’s Updated and Deleted handlers. The code will look like the following:

using System.Data;

protected void CoursesEntityDataSource_Updated(object sender,
EntityDataSourceChangedEventArgs e)
{
CheckForOptimisticConcurrencyException(e, "update");
}

protected void CoursesEntityDataSource_Deleted(object sender,
EntityDataSourceChangedEventArgs e)
{
CheckForOptimisticConcurrencyException(e, "delete");
}

private void
CheckForOptimisticConcurrencyException(EntityDataSourceChangedEventArgs e, string
function)
{
if (e.Exception != null && e.Exception is OptimisticConcurrencyException)
{
var concurrencyExceptionValidator = new CustomValidator();
concurrencyExceptionValidator.IsValid = false;
concurrencyExceptionValidator.ErrorMessage =
"The record you attempted to edit or delete was modified by
another " +
"user after you got the original value. The edit or delete
operation was canceled " +
"and the other user's values have been displayed so you can " +
"determine whether you still want to edit or delete this
record.";
Page.Validators.Add(concurrencyExceptionValidator);
e.ExceptionHandled = true;
}
}

Click here to view code as image

The only difference between this code and what you did for the ObjectDataSource control is that in this case the concurrency exception is in the Exception property of the event arguments object rather than in that exception’s InnerException property.

Run the page and create a concurrency conflict again. This time you see an error message:

This completes the introduction to handling concurrency conflicts. The next tutorial will provide guidance on how to improve performance in a web application that uses the Entity Framework.

Maximizing Performance

In the previous tutorial, you saw how to handle concurrency conflicts. This tutorial shows options for improving the performance of an ASP.NET web application that uses the Entity Framework. You’ll learn several methods for maximizing performance or for diagnosing performance problems.

Information presented in the following sections is likely to be useful in a broad variety of scenarios:

• Efficiently load related data.

• Manage view state.

Information presented in the following sections might be useful if you have individual queries that present performance problems:

• Use the NoTracking merge option.

• Pre-compile LINQ queries.

• Examine query commands sent to the database.

Information presented in the following section is potentially useful for applications that have extremely large data models:

• Pre-generate views.

Note Web application performance is affected by many factors, including things like the size of request and response data, the speed of database queries, how many requests the server can queue and how quickly it can service them, and even the efficiency of any client-script libraries you might be using. If performance is critical in your application, or if testing or experience shows that application performance isn’t satisfactory, you should follow normal protocol for performance tuning. Measure to determine where performance bottlenecks are occurring, and then address the areas that will have the greatest impact on overall application performance.

This topic focuses mainly on ways in which you can potentially improve the performance specifically of the Entity Framework in ASP.NET. The suggestions here are useful if you determine that data access is one of the performance bottlenecks in your application. Except as noted, the methods explained here shouldn’t be considered “best practices” in general — many of them are appropriate only in exceptional situations or to address very specific kinds of performance bottlenecks.

To start the tutorial, start Visual Studio and open the Contoso University web application that you were working with in the previous tutorial.

Efficiently Loading Related Data

There are several ways that the Entity Framework can load related data into the navigation properties of an entity:

• Lazy loading. When the entity is first read, related data isn’t retrieved. However, the first time you attempt to access a navigation property, the data required for that navigation property is automatically retrieved. This results in multiple queries sent to the database — one for the entity itself and one each time that related data for the entity must be retrieved.

Eager loading. When the entity is read, related data is retrieved along with it. This typically results in a single join query that retrieves all of the data that’s needed. You specify eager loading by using the Include method, as you’ve seen already in these tutorials.

• Explicit loading. This is similar to lazy loading, except that you explicitly retrieve the related data in code; it doesn’t happen automatically when you access a navigation property. You load related data manually using the Load method of the navigation property for collections, or you use the Load method of the reference property for properties that hold a single object. (For example, you call the PersonReference.Load method to load the Person navigation property of a Department entity.)

Because they don’t immediately retrieve the property values, lazy loading and explicit loading are also both known as deferred loading.

Lazy loading is the default behavior for an object context that has been generated by the designer. If you open the SchoolModel.Designer.cs file that defines the object context class, you’ll find three constructor methods, and each of them includes the following statement:

this.ContextOptions.LazyLoadingEnabled = true;

Click here to view code as image

In general, if you know you need related data for every entity retrieved, eager loading offers the best performance, because a single query sent to the database is typically more efficient than separate queries for each entity retrieved. On the other hand, if you need to access an entity’s navigation properties only infrequently or only for a small set of the entities, lazy loading or explicit loading may be more efficient, because eager loading would retrieve more data than you need.

In a web application, lazy loading may be of relatively little value anyway, because user actions that affect the need for related data take place in the browser, which has no connection to the object context that rendered the page. On the other hand, when you databind a control, you typically know what data you need, and so it’s generally best to choose eager loading or deferred loading based on what’s appropriate in each scenario.

In addition, a databound control might use an entity object after the object context is disposed. In that case, an attempt to lazy-load a navigation property would fail. The error message you receive is clear: “The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.”

The EntityDataSource control disables lazy loading by default. For the ObjectDataSource control that you’re using for the current tutorial (or if you access the object context from page code), there are several ways you can make lazy loading disabled by default. You can disable it when you instantiate an object context. For example, you can add the following line to the constructor method of the SchoolRepository class:

context.ContextOptions.LazyLoadingEnabled = false;

Click here to view code as image

For the Contoso University application, you’ll make the object context automatically disable lazy loading so that this property doesn’t have to be set whenever a context is instantiated.

Open the SchoolModel.edmx data model, click the design surface, and then in the properties pane set the Lazy Loading Enabled property to False. Save and close the data model.

Managing View State

In order to provide update functionality, an ASP.NET web page must store the original property values of an entity when a page is rendered. During postback processing the control can re-create the original state of the entity and call the entity’s Attach method before applying changes and calling the SaveChanges method. By default, ASP.NET Web Forms data controls use view state to store the original values. However, view state can affect performance, because it’s stored in hidden fields that can substantially increase the size of the page that’s sent to and from the browser.

Techniques for managing view state, or alternatives such as session state, aren’t unique to the Entity Framework, so this tutorial doesn’t go into this topic in detail. For more information see the links at the end of the tutorial.

However, version 4 of ASP.NET provides a new way of working with view state that every ASP.NET developer of Web Forms applications should be aware of: the ViewStateMode property. This new property can be set at the page or control level, and it enables you to disable view state by default for a page and enable it only for controls that need it.

For applications where performance is critical, a good practice is to always disable view state at the page level and enable it only for controls that require it. The size of view state in the Contoso University pages wouldn’t be substantially decreased by this method, but to see how it works, you’ll do it for the Instructors.aspx page. That page contains many controls, including a Label control that has view state disabled. None of the controls on this page actually need to have view state enabled. (The DataKeyNames property of the GridView control specifies state that must be maintained between postbacks, but these values are kept in control state, which isn’t affected by the ViewStateMode property.)

The Page directive and Label control markup currently resembles the following example:

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true"
CodeBehind="Instructors.aspx.cs" Inherits="ContosoUniversity.Instructors" %>
...
<asp:Label ID="ErrorMessageLabel" runat="server" Text="" Visible="false"
ViewStateMode="Disabled"></asp:Label>
...

Click here to view code as image

Make the following changes:

• Add ViewStateMode="Disabled" to the Page directive.

• Remove ViewStateMode="Disabled" from the Label control.

The markup now resembles the following example:

<%@ Page Title="" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true"
CodeBehind="Instructors.aspx.cs" Inherits="ContosoUniversity.Instructors"
ViewStateMode="Disabled" %>
...
<asp:Label ID="ErrorMessageLabel" runat="server" Text=""
Visible="false"></asp:Label>
...

Click here to view code as image

View state is now disabled for all controls. If you later add a control that does need to use view state, all you need to do is include the ViewStateMode="Enabled" attribute for that control.

Using The NoTracking Merge Option

When an object context retrieves database rows and creates entity objects that represent them, by default it also tracks those entity objects using its object state manager. This tracking data acts as a cache and is used when you update an entity. Because a web application typically has short-lived object context instances, queries often return data that doesn’t need to be tracked, because the object context that reads them will be disposed before any of the entities it reads are used again or updated.

In the Entity Framework, you can specify whether the object context tracks entity objects by setting a merge option. You can set the merge option for individual queries or for entity sets. If you set it for an entity set, that means that you’re setting the default merge option for all queries that are created for that entity set.

For the Contoso University application, tracking isn’t needed for any of the entity sets that you access from the repository, so you can set the merge option to NoTracking for those entity sets when you instantiate the object context in the repository class. (Note that in this tutorial, setting the merge option won’t have a noticeable effect on the application’s performance. The NoTracking option is likely to make an observable performance improvement only in certain high-data-volume scenarios.)

In the DAL folder, open the SchoolRepository.cs file and add a constructor method that sets the merge option for the entity sets that the repository accesses:

public SchoolRepository()
{
context.Departments.MergeOption = MergeOption.NoTracking;
context.InstructorNames.MergeOption = MergeOption.NoTracking;
context.OfficeAssignments.MergeOption = MergeOption.NoTracking;
}

Click here to view code as image

Pre-Compiling LINQ Queries

The first time that the Entity Framework executes an Entity SQL query within the life of a given ObjectContext instance, it takes some time to compile the query. The result of compilation is cached, which means that subsequent executions of the query are much quicker. LINQ queries follow a similar pattern, except that some of the work required to compile the query is done every time the query is executed. In other words, for LINQ queries, by default not all of the results of compilation are cached.

If you have a LINQ query that you expect to run repeatedly in the life of an object context, you can write code that causes all of the results of compilation to be cached the first time the LINQ query is run.

As an illustration, you’ll do this for two Get methods in the SchoolRepository class, one of which doesn’t take any parameters (the GetInstructorNames method), and one that does require a parameter (the GetDepartmentsByAdministrator method). These methods as they stand now actually don’t need to be compiled because they aren’t LINQ queries:

public IEnumerable<InstructorName> GetInstructorNames()
{
return context.InstructorNames.OrderBy("it.FullName").ToList();
}

public IEnumerable<Department> GetDepartmentsByAdministrator(Int32
administrator)
{
return new ObjectQuery<Department>("SELECT VALUE d FROM Departments as
d", context, MergeOption.NoTracking).Include("Person").Where(d => d.Administrator ==
administrator).ToList();
}

Click here to view code as image

However, so that you can try out compiled queries, you’ll proceed as if these had been written as the following LINQ queries:

public IEnumerable<InstructorName> GetInstructorNames()
{
return (from i in context.InstructorNames orderby i.FullName select
i).ToList();
}

Click here to view code as image

public IEnumerable<Department> GetDepartmentsByAdministrator(Int32
administrator)
{
context.Departments.MergeOption = MergeOption.NoTracking;
return (from d in context.Departments where d.Administrator ==
administrator select d).ToList();
}

Click here to view code as image

You could change the code in these methods to what’s shown above and run the application to verify that it works before continuing. But the following instructions jump right into creating pre-compiled versions of them.

Create a class file in the DAL folder, name it SchoolEntities.cs, and replace the existing code with the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Data.Objects;

namespace ContosoUniversity.DAL
{
public partial class SchoolEntities
{
private static readonly Func<SchoolEntities, IQueryable<InstructorName>>
compiledInstructorNamesQuery =
CompiledQuery.Compile((SchoolEntities context) => from i in
context.InstructorNames orderby i.FullName select i);

public IEnumerable<InstructorName> CompiledInstructorNamesQuery()
{
return compiledInstructorNamesQuery(this).ToList();
}

private static readonly Func<SchoolEntities, Int32, IQueryable<Department>>
compiledDepartmentsByAdministratorQuery =
CompiledQuery.Compile((SchoolEntities context, Int32 administrator) =>
from d in context.Departments.Include("Person") where d.Administrator ==
administrator select d);

public IEnumerable<Department> CompiledDepartmentsByAdministratorQuery(Int32
administrator)
{
return compiledDepartmentsByAdministratorQuery(this,
administrator).ToList();
}
}
}

Click here to view code as image

This code creates a partial class that extends the automatically generated object context class. The partial class includes two compiled LINQ queries using the Compile method of the CompiledQuery class. It also creates methods that you can use to call the queries. Save and close this file.

Next, in SchoolRepository.cs, change the existing GetInstructorNames and GetDepartmentsByAdministrator methods in the repository class so that they call the compiled queries:

public IEnumerable<InstructorName> GetInstructorNames()
{
return context.CompiledInstructorNamesQuery();
}

public IEnumerable<Department> GetDepartmentsByAdministrator(Int32
administrator)
{
return context.CompiledDepartmentsByAdministratorQuery(administrator);
}

Click here to view code as image

Run the Departments.aspx page to verify that it works as it did before. The GetInstructorNames method is called in order to populate the administrator drop-down list, and the GetDepartmentsByAdministrator method is called when you click Update in order to verify that no instructor is an administrator of more than one department.

You’ve pre-compiled queries in the Contoso University application only to see how to do it, not because it would measurably improve performance. Pre-compiling LINQ queries does add a level of complexity to your code, so make sure you do it only for queries that actually represent performance bottlenecks in your application.

Examining Queries Sent to the Database

When you’re investigating performance issues, sometimes it’s helpful to know the exact SQL commands that the Entity Framework is sending to the database. If you’re working with an IQueryable object, one way to do this is to use the ToTraceString method.

In SchoolRepository.cs, change the code in the GetDepartmentsByName method to match the following example:

public IEnumerable<Department> GetDepartmentsByName(string sortExpression,
string nameSearchString)
{
...
var departments = new ObjectQuery<Department>("SELECT VALUE d FROM
Departments AS d", context).OrderBy("it." +
sortExpression).Include("Person").Include("Courses").Where(d =>
d.Name.Contains(nameSearchString));
string commandText = ((ObjectQuery)departments).ToTraceString();
return departments.ToList();
}

Click here to view code as image

The departments variable must be cast to an ObjectQuery type only because the Where method at the end of the preceding line creates an IQueryable object; without the Where method, the cast would not be necessary.

Set a breakpoint on the return line, and then run the Departments.aspx page in the debugger. When you hit the breakpoint, examine the commandText variable in the Locals window and use the text visualizer (the magnifying glass in the Value column) to display its value in the Text Visualizer window. You can see the entire SQL command that results from this code:

As an alternative, the IntelliTrace feature in Visual Studio Ultimate provides a way to view SQL commands generated by the Entity Framework that doesn’t require you to change your code or even set a breakpoint.

Note You can perform the following procedures only if you have Visual Studio Ultimate.

Restore the original code in the GetDepartmentsByName method, and then run the Departments.aspx page in the debugger.

In Visual Studio, select the Debug menu, then IntelliTrace, and then IntelliTrace Events.

In the IntelliTrace window, click Break All.

The IntelliTrace window displays a list of recent events:

Click the ADO.NET line. It expands to show you the command text:

You can copy the entire command text string to the clipboard from the Locals window.

Suppose you were working with a database with more tables, relationships, and columns than the simple School database. You might find that a query that gathers all the information you need in a single Select statement containing multiple Join clauses becomes too complex to work efficiently. In that case you can switch from eager loading to explicit loading to simplify the query.

For example, try changing the code in the GetDepartmentsByName method in SchoolRepository.cs. Currently in that method you have an object query that has Include methods for the Person and Courses navigation properties. Replace the return statement with code that performs explicit loading, as shown in the following example:

public IEnumerable<Department> GetDepartmentsByName(string sortExpression,
string nameSearchString)
{
...
var departments = new ObjectQuery<Department>("SELECT VALUE d FROM
Departments AS d", context).OrderBy("it." + sortExpression).Where(d =>
d.Name.Contains(nameSearchString)).ToList();
foreach (Department d in departments)
{
d.Courses.Load();
d.PersonReference.Load();
}
return departments;
}

Click here to view code as image

Run the Departments.aspx page in the debugger and check the IntelliTrace window again as you did before. Now, where there was a single query before, you see a long sequence of them.

Click the first ADO.NET line to see what has happened to the complex query you viewed earlier.

The query from Departments has become a simple Select query with no Join clause, but it’s followed by separate queries that retrieve related courses and an administrator, using a set of two queries for each department returned by the original query.

Note If you leave lazy loading enabled, the pattern you see here, with the same query repeated many times, might result from lazy loading. A pattern that you typically want to avoid is lazy-loading related data for every row of the primary table. Unless you’ve verified that a single join query is too complex to be efficient, you’d typically be able to improve performance in such cases by changing the primary query to use eager loading.

Pre-Generating Views

When an ObjectContext object is first created in a new application domain, the Entity Framework generates a set of classes that it uses to access the database. These classes are called views, and if you have a very large data model, generating these views can delay the web site’s response to the first request for a page after a new application domain is initialized. You can reduce this first-request delay by creating the views at compile time rather than at run time.

Note If your application doesn’t have an extremely large data model, or if it does have a large data model but you aren’t concerned about a performance problem that affects only the very first page request after IIS is recycled, you can skip this section. View creation doesn’t happen every time you instantiate an ObjectContext object, because the views are cached in the application domain. Therefore, unless you’re frequently recycling your application in IIS, very few page requests would benefit from pre-generated views.

You can pre-generate views using the EdmGen.exe command-line tool or by using a Text Template Transformation Toolkit (T4) template. In this tutorial you’ll use a T4 template.

In the DAL folder, add a file using the Text Template template (it’s under the General node in the Installed Templates list), and name it SchoolModel.Views.tt. Replace the existing code in the file with the following code:

<#
/***************************************************************************

Copyright (c) Microsoft Corporation. All rights reserved.

THIS CODE IS PROVIDED *AS IS* WITHOUT WARRANTY OF
ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING ANY
IMPLIED WARRANTIES OF FITNESS FOR A PARTICULAR
PURPOSE, MERCHANTABILITY, OR NON-INFRINGEMENT.

***************************************************************************/
#>

<#
//
// TITLE: T4 template to generate views for an EDMX file in a C# project
//
// DESCRIPTION:
// This is a T4 template to generate views in C# for an EDMX file in C# projects.
// The generated views are automatically compiled into the project's output
assembly.
//
// This template follows a simple file naming convention to determine the EDMX
file to process:
// - It assumes that [edmx-file-name].Views.tt will process and generate views
for [edmx-file-name].EDMX
// - The views are generated in the code behind file [edmx-file-name].Views.cs
//
// USAGE:
// Do the following to generate views for an EDMX file (e.g. Model1.edmx) in a C#
project
// 1. In Solution Explorer, right-click the project node and choose
"Add...Existing...Item" from the context menu
// 2. Browse to and choose this .tt file to include it in the project
// 3. Ensure this .tt file is in the same directory as the EDMX file to process
// 4. In Solution Explorer, rename this .tt file to the form [edmx-file-
name].Views.tt (e.g. Model1.Views.tt)
// 5. In Solution Explorer, right-click Model1.Views.tt and choose "Run Custom
Tool" to generate the views
// 6. The views are generated in the code behind file Model1.Views.cs
//
// TIPS:
// If you have multiple EDMX files in your project then make as many copies of
this .tt file and rename appropriately
// to pair each with each EDMX file.
//
// To generate views for all EDMX files in the solution, click the "Transform All
Templates" button in the Solution Explorer toolbar
// (its the rightmost button in the toolbar)
//
#>
<#
//
// T4 template code follows
//
#>
<#@ template language="C#" hostspecific="true"#>
<#@ include file="EF.Utility.CS.ttinclude"#>
<#@ output extension=".cs" #>
<#
// Find EDMX file to process: Model1.Views.tt generates views for Model1.EDMX
string edmxFileName =
Path.GetFileNameWithoutExtension(this.Host.TemplateFile).ToLowerInvariant().Replace("
.views", "") + ".edmx";
string edmxFilePath = Path.Combine(Path.GetDirectoryName(this.Host.TemplateFile),
edmxFileName);
if (File.Exists(edmxFilePath))
{
// Call helper class to generate pre-compiled views and write to output
this.WriteLine(GenerateViews(edmxFilePath));
}
else
{
this.Error(String.Format("No views were generated. Cannot find file {0}.
Ensure the project has an EDMX file and the file name of the .tt file is of the form
[edmx-file-name].Views.tt", edmxFilePath));
}

// All done!
#>

<#+
private String GenerateViews(string edmxFilePath)
{
MetadataLoader loader = new MetadataLoader(this);
MetadataWorkspace workspace;
if(!loader.TryLoadAllMetadata(edmxFilePath, out workspace))
{
this.Error("Error in the metadata");
return String.Empty;
}

String generatedViews = String.Empty;
try
{
using (StreamWriter writer = new StreamWriter(new MemoryStream()))
{
StorageMappingItemCollection mappingItems =
(StorageMappingItemCollection)workspace.GetItemCollection(DataSpace.CSSpace);

// Initialize the view generator to generate views in C#
EntityViewGenerator viewGenerator = new EntityViewGenerator();
viewGenerator.LanguageOption = LanguageOption.GenerateCSharpCode;
IList<EdmSchemaError> errors =
viewGenerator.GenerateViews(mappingItems, writer);

foreach (EdmSchemaError e in errors)
{
// log error
this.Error(e.Message);
}

MemoryStream memStream = writer.BaseStream as MemoryStream;
generatedViews = Encoding.UTF8.GetString(memStream.ToArray());
}
}
catch (Exception ex)
{
// log error
this.Error(ex.ToString());
}

return generatedViews;
}
#>

Click here to view code as image

This code generates views for an .edmx file that’s located in the same folder as the template and that has the same name as the template file. For example, if your template file is named SchoolModel.Views.tt, it will look for a data model file named SchoolModel.edmx.

Save the file, then right-click the file in Solution Explorer and select Run Custom Tool.

Visual Studio generates a code file that creates the views, which is named SchoolModel.Views.cs based on the template. (You might have noticed that the code file is generated even before you select Run Custom Tool, as soon as you save the template file.)

You can now run the application and verify that it works as it did before.

For more information about pre-generated views, see the following resources:

• How to: Pre-Generate Views to Improve Query Performance on the MSDN web site. Explains how to use the EdmGen.exe command-line tool to pre-generate views.

• Isolating Performance with Precompiled/Pre-generated Views in the Entity Framework 4 on the Windows Server AppFabric Customer Advisory Team blog.

This completes the introduction to improving performance in an ASP.NET web application that uses the Entity Framework. For more information, see the following resources:

• Performance Considerations (Entity Framework) on the MSDN web site.

• Performance-related posts on the Entity Framework Team blog.

• EF Merge Options and Compiled Queries. Blog post that explains unexpected behaviors of compiled queries and merge options such as NoTracking. If you plan to use compiled queries or manipulate merge option settings in your application, read this first.

• Entity Framework-related posts in the Data and Modeling Customer Advisory Team blog. Includes posts on compiled queries and using the Visual Studio 2010 Profiler to discover performance issues.

• Entity Framework forum thread with advice on improving performance of highly complex queries.

• ASP.NET State Management Recommendations.

• Using the Entity Framework and the ObjectDataSource: Custom Paging. Blog post that builds on the ContosoUniversity application created in these tutorials to explain how to implement paging in the Departments.aspx page.

The next tutorial reviews some of the important enhancements to the Entity Framework that are new in version 4.

What’s New in the Entity Framework 4

By Tom Dykstra|January 26, 2011

This tutorial series builds on the Contoso University web application that is created by the Getting Started with the Entity Framework tutorial series. If you didn’t complete the earlier tutorials, as a starting point for this tutorial you can download the application that you would have created. You can also download the application that is created by the complete tutorial series. If you have questions about the tutorials, you can post them to the ASP.NET Entity Framework forum.

In the previous tutorial you saw some methods for maximizing the performance of a web application that uses the Entity Framework. This tutorial reviews some of the most important new features in version 4 of the Entity Framework, and it links to resources that provide a more complete introduction to all of the new features. The features highlighted in this tutorial include the following:

• Foreign-key associations.

• Executing user-defined SQL commands.

• Model-first development.

• POCO support.

In addition, the tutorial will briefly introduce code-first development, a feature that’s coming in the next release of the Entity Framework.

To start the tutorial, start Visual Studio and open the Contoso University web application that you were working with in the previous tutorial.

Foreign-Key Associations

Version 3.5 of the Entity Framework included navigation properties, but it didn’t include foreign-key properties in the data model. For example, the CourseID and StudentID columns of the StudentGrade table would be omitted from the StudentGrade entity.

The reason for this approach was that, strictly speaking, foreign keys are a physical implementation detail and don’t belong in a conceptual data model. However, as a practical matter, it’s often easier to work with entities in code when you have direct access to the foreign keys.

For an example of how foreign keys in the data model can simplify your code, consider how you would have had to code the DepartmentsAdd.aspx page without them. In the Department entity, the Administrator property is a foreign key that corresponds to PersonID in the Person entity. In order to establish the association between a new department and its administrator, all you had to do was set the value for the Administrator property in the ItemInserting event handler of the databound control:

protected void DepartmentsDetailsView_ItemInserting(object sender,
DetailsViewInsertEventArgs e)
{
e.Values["Administrator"] = administratorsDropDownList.SelectedValue;
}

Click here to view code as image

Without foreign keys in the data model, you’d handle the Inserting event of the data source control instead of the ItemInserting event of the databound control, in order to get a reference to the entity itself before the entity is added to the entity set. When you have that reference, you establish the association using code like that in the following examples:

departmentEntityToBeInserted.PersonReference.EntityKey = new
System.Data.EntityKey("SchoolEntities.Departments", "PersonID",
Convert.ToInt32(administratorsDropDownList.SelectedValue));

Click here to view code as image

departmentEntityToBeInserted.Person = context.People.Single(p =>
p.PersonID == Convert.ToInt32(administratorsDropDownList.SelectedValue));

Click here to view code as image

As you can see in the Entity Framework team’s blog post on Foreign Key associations, there are other cases where the difference in code complexity is much greater. To meet the needs of those who prefer to live with implementation details in the conceptual data model for the sake of simpler code, the Entity Framework now gives you the option of including foreign keys in the data model.

In Entity Framework terminology, if you include foreign keys in the data model you’re using foreign key associations, and if you exclude foreign keys you’re using independent associations.

Executing User-Defined SQL Commands

In earlier versions of the Entity Framework, there was no easy way to create your own SQL commands on the fly and run them. Either the Entity Framework dynamically generated SQL commands for you, or you had to create a stored procedure and import it as a function. Version 4 adds ExecuteStoreQuery and ExecuteStoreCommand methods the ObjectContext class that make it easier for you to pass any query directly to the database.

Suppose Contoso University administrators want to be able to perform bulk changes in the database without having to go through the process of creating a stored procedure and importing it into the data model. Their first request is for a page that lets them change the number of credits for all courses in the database. On the web page, they want to be able to enter a number to use to multiply the value of every Course row’s Credits column.

Create a new page that uses the Site.Master master page and name it UpdateCredits.aspx. Then add the following markup to the Content control named Content2:

<h2>Update Credits</h2>
Enter the number to multiply the current number of credits by:
<asp:TextBox ID="CreditsMultiplierTextBox" runat="server"></asp:TextBox>
<br /><br />
<asp:Button ID="ExecuteButton" runat="server" Text="Execute"
OnClick="ExecuteButton_Click" /><br /><br />
Rows affected:
<asp:Label ID="RowsAffectedLabel" runat="server" Text="0"
ViewStateMode="Disabled"></asp:Label><br /><br />

Click here to view code as image

This markup creates a TextBox control in which the user can enter the multiplier value, a Button control to click in order to execute the command, and a Label control for indicating the number of rows affected.

Open UpdateCredits.aspx.cs, and add the following using statement and a handler for the button’s Click event:

using ContosoUniversity.DAL;

protected void ExecuteButton_Click(object sender, EventArgs e)
{
using (SchoolEntities context = new SchoolEntities())
{
RowsAffectedLabel.Text = context.ExecuteStoreCommand("UPDATE Course
SET Credits = Credits * {0}", CreditsMultiplierTextBox.Text).ToString();
}
}

Click here to view code as image

This code executes the SQL Update command using the value in the text box and uses the label to display the number of rows affected. Before you run the page, run the Courses.aspx page to get a “before” picture of some data.

Run UpdateCredits.aspx, enter “10” as the multiplier, and then click Execute.

Run the Courses.aspx page again to see the changed data.

(If you want to set the number of credits back to their original values, in UpdateCredits.aspx.cs change Credits * {0} to Credits / {0} and re-run the page, entering 10 as the divisor.)

For more information about executing queries that you define in code, see How to: Directly Execute Commands Against the Data Source.

Model-First Development

In these walkthroughs you created the database first and then generated the data model based on the database structure. In the Entity Framework 4 you can start with the data model instead and generate the database based on the data model structure. If you’re creating an application for which the database doesn’t already exist, the model-first approach enables you to create entities and relationships that make sense conceptually for the application, while not worrying about physical implementation details. (This remains true only through the initial stages of development, however. Eventually the database will be created and will have production data in it, and recreating it from the model will no longer be practical; at that point you’ll be back to the database-first approach.)

In this section of the tutorial, you’ll create a simple data model and generate the database from it.

In Solution Explorer, right-click the DAL folder and select Add New Item. In the Add New Item dialog box, under Installed Templates select Data and then select the ADO.NET Entity Data Model template. Name the new file AlumniAssociationModel.edmx and click Add.

This launches the Entity Data Model Wizard. In the Choose Model Contents step, select Empty Model and then click Finish.

The Entity Data Model Designer opens with a blank design surface. Drag an Entity item from the Toolbox onto the design surface.

Change the entity name from Entity1 to Alumnus, change the Id property name to AlumnusId, and add a new scalar property named Name. To add new properties you can press Enter after changing the name of the Id column, or right-click the entity and select Add Scalar Property. The default type for new properties is String, which is fine for this simple demonstration, but of course you can change things like data type in the Properties window.

Create another entity the same way and name it Donation. Change the Id property to DonationId and add a scalar property named DateAndAmount.

To add an association between these two entities, right-click the Alumnus entity, select Add, and then select Association.

The default values in the Add Association dialog box are what you want (one-to-many, include navigation properties, include foreign keys), so just click OK.

The designer adds an association line and a foreign-key property.

Now you’re ready to create the database. Right-click the design surface and select Generate Database from Model.

This launches the Generate Database Wizard. (If you see warnings that indicate that the entities aren’t mapped, you can ignore those for the time being.)

In the Choose Your Data Connection step, click New Connection.

In the Connection Properties dialog box, select the local SQL Server Express instance and name the database AlumniAsssociation.

Click Yes when you’re asked if you want to create the database. When the Choose Your Data Connection step is displayed again, click Next.

In the Summary and Settings step, click Finish.

A .sql file with the data definition language (DDL) commands is created, but the commands haven’t been run yet.

Use a tool such as SQL Server Management Studio to run the script and create the tables, as you might have done when you created the School database for the first tutorial. (Unless you downloaded the database.)

You can now use the AlumniAssociation data model in your web pages the same way you’ve been using the School model. To try this out, add some data to the tables and create a web page that displays the data.

Using Server Explorer, add the following rows to the Alumnus and Donation tables.

Create a new web page named Alumni.aspx that uses the Site.Master master page. Add the following markup to the Content control named Content2:

<h2>Alumni</h2>
<asp:EntityDataSource ID="AlumniEntityDataSource" runat="server"
ContextTypeName="ContosoUniversity.DAL.AlumniAssociationModelContainer"
EnableFlattening="False"
EntitySetName="Alumni">
</asp:EntityDataSource>
<asp:GridView ID="AlumniGridView" runat="server"
DataSourceID="AlumniEntityDataSource" AutoGenerateColumns="False"
OnRowDataBound="AlumniGridView_RowDataBound"
DataKeyNames="AlumnusId">
<Columns>
<asp:BoundField DataField="Name" HeaderText="Name" SortExpression="Name"
/>
<asp:TemplateField HeaderText="Donations">
<ItemTemplate>
<asp:GridView ID="DonationsGridView" runat="server"
AutoGenerateColumns="False">
<Columns>
<asp:BoundField DataField="DateAndAmount"
HeaderText="Date and Amount" />
</Columns>
</asp:GridView>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

Click here to view code as image

This markup creates nested GridView controls, the outer one to display alumni names and the inner one to display donation dates and amounts.

Open Alumni.aspx.cs. Add a using statement for the data access layer and a handler for the outer GridView control’s RowDataBound event:

using ContosoUniversity.DAL;

// ...
protected void AlumniGridView_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow)
{
var alumnus = e.Row.DataItem as Alumnus;
var donationsGridView = (GridView)e.Row.FindControl("DonationsGridView");
donationsGridView.DataSource = alumnus.Donations.ToList();
donationsGridView.DataBind();
}
}

Click here to view code as image

This code databinds the inner GridView control using the Donations navigation property of the current row’s Alumnus entity.

Run the page.

(Note: This page is included in the downloadable project, but to make it work you must create the database in your local SQL Server Express instance; the database isn’t included as an .mdf file in the App_Data folder.)

For more information about using the model-first feature of the Entity Framework, see Model-First in the Entity Framework 4.

POCO Support

When you use domain-driven design methodology, you design data classes that represent data and behavior that’s relevant to the business domain. These classes should be independent of any specific technology used to store (persist) the data; in other words, they should be persistence ignorant. Persistence ignorance can also make a class easier to unit test because the unit test project can use whatever persistence technology is most convenient for testing. Earlier versions of the Entity Framework offered limited support for persistence ignorance because entity classes had to inherit from the EntityObject class and thus included a great deal of Entity Framework-specific functionality.

The Entity Framework 4 introduces the ability to use entity classes that don’t inherit from the EntityObject class and therefore are persistence ignorant. In the context of the Entity Framework, classes like this are typically called plain-old CLR objects (POCO, or POCOs). You can write POCO classes manually, or you can automatically generate them based on an existing data model using Text Template Transformation Toolkit (T4) templates provided by the Entity Framework.

For more information about using POCOs in the Entity Framework, see the following resources:

• Working with POCO Entities. This is an MSDN document that’s an overview of POCOs, with links to other documents that have more detailed information.

• Walkthrough: POCO Template for the Entity Framework This is a blog post from the Entity Framework development team, with links to other blog posts about POCOs.

Code-First Development

POCO support in the Entity Framework 4 still requires that you create a data model and link your entity classes to the data model. The next release of the Entity Framework will include a feature called code-first development. This feature enables you to use the Entity Framework with your own POCO classes without needing to use either the data model designer or a data model XML file. (Therefore, this option has also been called code-only; code-first and code-only both refer to the same Entity Framework feature.)

For more information about using the code-first approach to development, see the following resources:

• Getting Started with Entity Framework Using MVC

• Code-First Development with Entity Framework 4. This is a blog post by Scott Guthrie introducing code-first development.

• Entity Framework Development Team Blog - posts tagged CodeOnly

• Entity Framework Development Team Blog - posts tagged Code First

• MVC Music Store tutorial - Part 4: Models and Data Access

• Getting Started with MVC 3 - Part 4: Entity Framework Code-First Development

More Information

This concludes this series of tutorials on Getting Started with the Entity Framework. For more resources to help you learn how to use the Entity Framework, continue with the first tutorial in the next Entity Framework tutorial series or visit the following sites:

• Entity Framework FAQ

• The Entity Framework Team Blog

• Entity Framework in the MSDN Library

• Entity Framework in the MSDN Data Developer Center

• EntityDataSource Web Server Control Overview in the MSDN Library

• EntityDataSource control API reference in the MSDN Library

• Entity Framework Forums on MSDN

• Julie Lerman’s blog

• What’s New in ADO.NET MSDN topic on new features in version 4 of the Entity Framework.

• Announcing the release of Entity Framework 4 The Entity Framework development team’s blog post about new features in version 4.

Appendix: Code Images

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