Walkthrough: Creating LINQ to SQL Classes by Using Single-Table Inheritance (O/R Designer)

Article
02/23/2011

The Object Relational Designer (O/R Designer) supports single-table inheritance as it is typically implemented in relational systems. This walkthrough expands upon the generic steps provided in the How to: Configure Inheritance by Using the O/R Designer topic and provides some real data to demonstrate the use of inheritance in the O/R Designer.

During this walkthrough, you will perform the following tasks:

Create a database table and add data to it.
Create a Windows Forms application.
Add a LINQ to SQL file to a project.
Create new entity classes.
Configure the entity classes to use inheritance.
Query the inherited class.
Display the data on a Windows Form.

Create a Table to Inherit From

To see how inheritance works, you will create a small Person table, use it as a base class, and then create an Employee object that inherits from it.

To create a base table to demonstrate inheritance

In Server Explorer/Database Explorer, right-click the Tables node and click Add New Table.

Note

You can use the Northwind database or any other database that you can add a table to.
In the Table Designer, add the following columns to the table:

Column Name

Data Type

Allow Nulls

ID

int

False

Type

int

True

FirstName

nvarchar(200)

False

LastName

nvarchar(200)

False

Manager

int

True
Set the ID column as the primary key.
Save the table and name it Person.

Column Name	Data Type	Allow Nulls
ID	int	False
Type	int	True
FirstName	nvarchar(200)	False
LastName	nvarchar(200)	False
Manager	int	True

Add Data to the Table

So that you can verify that inheritance is configured correctly, the table needs some data for each class in the single-table inheritance.

To add data to the table

Open the table in data view. (Right-click the Person table in Server Explorer/Database Explorer and click Show Table Data.)

Copy the following data into the table. (You can copy it and then paste it into the table by selecting the whole row in the Results Pane.)

ID	Type	FirstName	LastName	Manager
1	1	Anne	Wallace	NULL
2	1	Carlos	Grilo	NULL
3	1	Yael	Peled	NULL
4	2	Gatis	Ozolins	1
5	2	Andreas	Hauser	1
6	2	Tiffany	Phuvasate	1
7	2	Alexey	Orekhov	2
8	2	Michał	Poliszkiewicz	2
9	2	Tai	Yee	2
10	2	Fabricio	Noriega	3
11	2	Mindy	Martin	3
12	2	Ken	Kwok	3

Create a New Project

Now that you have created the table, create a new project to demonstrate configuring inheritance.

To create the new Windows Application

From the File menu, create a new project.
Name the project InheritanceWalkthrough.

Note

The O/R Designer is supported in Visual Basic and C# projects. Create the new project in one of these languages.
Click the Windows Forms Application template and then click OK. For more information, see Creating Windows-Based Applications.
The InheritanceWalkthrough project is created and added to Solution Explorer.

Add a LINQ to SQL Classes File to the Project

To add a LINQ to SQL File to the project

On the Project menu, click Add New Item.
Click the LINQ to SQL Classes template and then click Add.

The .dbml file is added to the project and the O/R Designer opens.

Create the Inheritance by Using the O/R Designer

Configure the inheritance by dragging an Inheritance object from the Toolbox onto the design surface.

To create the inheritance

In Server Explorer/Database Explorer, navigate to the Person table that you created earlier.
Drag the Person table onto the O/R Designer design surface.
Drag a second Person table onto the O/R Designer and change its name to Employee.
Delete the Manager property from the Person object.
Delete the Type, ID, FirstName, and LastName properties from the Employee object. (In other words, delete all properties except for Manager.)
From the Object Relational Designer tab of the Toolbox, create an Inheritance between the Person and Employee objects. To do this, click the Inheritance item in the Toolbox and release the mouse button. Next, click the Employee object and then the Person object in the O/R Designer. The arrow on the inheritance line will point to the Person object.
Click the Inheritance line on the design surface.
Set the Discriminator Property property to Type.
Set the Derived Class Discriminator Value property to 2.
Set the Base Class Discriminator Value property to 1.
Set the Inheritance Default property to Person.
Build the project.

Query the Inherited Class and Display the Data on the Form

You will now add some code to the form that queries for a specific class in the object model.

To create a LINQ query and display the results on the form

Drag a ListBox onto Form1.
Double-click the form to create a Form1_Load event handler.

Add the following code to the Form1_Load event handler:

Dim dc As New DataClasses1DataContext
Dim results = From emp In dc.Persons _
    Where TypeOf emp Is Employee _
    Select emp

For Each Emp As Employee In results
    ListBox1.Items.Add(Emp.LastName)
Next

NorthwindDataContext dc = new DataClasses1DataContext();
var results = from emp in dc.Persons
              where emp is Employee
              select emp;

foreach(Employee Emp in results)
{
    listBox1.Items.Add(Emp.LastName)
}

Test the Application

Run the application and verify that the records displayed in the list box are all employees (records that have a value of 2 in their Type column).

To test the application

Press F5.
Verify that only records that have a value of 2 in their Type column are displayed.
Close the form. (On the Debug menu, click Stop Debugging.)

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