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Walkthrough: Create LINQ to SQL classes by using single-table inheritance (O/R Designer)

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The LINQ to SQL tools in Visual Studio 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 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 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

  1. In Server Explorer or 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.

  2. 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

  3. Set the ID column as the primary key.

  4. Save the table and name it Person.

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

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

  2. 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 Forms application

  1. In Visual Studio, on the File menu, select New > Project.

  2. Expand either Visual C# or Visual Basic in the left-hand pane, then select Windows Desktop.

  3. In the middle pane, select the Windows Forms App project type.

  4. Name the project InheritanceWalkthrough, and then choose OK.

    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

  1. On the Project menu, click Add New Item.

  2. Click the LINQ to SQL Classes template, and then select 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

  1. In Server Explorer or Database Explorer, navigate to the Person table that you created earlier.

  2. Drag the Person table onto the O/R Designer design surface.

  3. Drag a second Person table onto the O/R Designer and change its name to Employee.

  4. Delete the Manager property from the Person object.

  5. Delete the Type, ID, FirstName, and LastName properties from the Employee object. (In other words, delete all properties except for Manager.)

  6. 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 then points to the Person object.

  7. Click the Inheritance line on the design surface.

  8. Set the Discriminator Property property to Type.

  9. Set the Derived Class Discriminator Value property to 2.

  10. Set the Base Class Discriminator Value property to 1.

  11. Set the Inheritance Default property to Person.

  12. Build the project.

Query the inherited class and display the data on the form

You 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

  1. Drag a ListBox onto Form1.

  2. Double-click the form to create a Form1_Load event handler.

  3. Add the following code to the Form1_Load event handler:

    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

  1. Press F5.

  2. Verify that only records that have a value of 2 in their Type column are displayed.

  3. Close the form. (On the Debug menu, click Stop Debugging.)

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