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Adding Business Logic By Using Partial Methods

You can customize Visual Basic and C# generated code in your LINQ to SQL projects by using partial methods. The code that LINQ to SQL generates defines signatures as one part of a partial method. If you want to implement the method, you can add your own partial method. If you do not add your own implementation, the compiler discards the partial methods signature and calls the default methods in LINQ to SQL. 

Note Note

If you are using Visual Studio, you can use the Object Relational Designer to add validation and other customizations to entity classes. How to: Add Validation to Entity Classes
How to: Add Validation to Entity Classes

For example, the default mapping for the Customer class in the Northwind sample database includes the following partial method:

Partial Private Sub OnAddressChanged()
End Sub

You can implement your own method by adding code such as the following to your own partial Customer class:

Partial Class Customer
    Private Sub OnAddressChanged()
        ' Insert business logic here. 
    End Sub 
End Class

This approach is typically used in LINQ to SQL to override default methods for Insert, Update, Delete, and to validate properties during object life-cycle events.

For more information, see Partial Methods (Visual Basic) (Visual Basic) or partial (Method) (C# Reference) (C#).

The following example shows ExampleClass first as it might be defined by a code-generating tool such as SQLMetal, and then how you might implement only one of the two methods.

' Code-generating tool defines a partial class, including  
' two partial methods.  
Partial Class ExampleClass
    Partial Private Sub OnFindingMaxOutput()
    End Sub 

    Partial Private Sub OnFindingMinOutput()
    End Sub 

    Sub ExportResults()
        OnFindingMaxOutput()
        OnFindingMinOutput()
    End Sub 
End Class 

' Developer implements one of the partial methods. Compiler 
' discards the other method. 
Class ExampleClass
    Private Sub OnFindingMaxOutput()
        Console.WriteLine("Maximum has been found.")
    End Sub 
End Class

The following example uses the relationship between Shipper and Order entities. Note among the methods the partial methods, InsertShipper and DeleteShipper. These methods override the default partial methods supplied by LINQ to SQL mapping.

Public Shared LoadOrdersCalled As Integer = 0
Private Function LoadOrders(ByVal shipper As Shipper) As  _
    IEnumerable(Of Order)
    LoadOrdersCalled += 1
    Return Me.Orders.Where(Function(o) o.ShipVia = _
        shipper.ShipperID)
End Function 

Public Shared LoadShipperCalled As Integer = 0
Private Function LoadShipper(ByVal order As Order) As Shipper
    LoadShipperCalled += 1
    Return Me.Shippers.Single(Function(s) s.ShipperID = _
        order.ShipVia)
End Function 

Public Shared InsertShipperCalled As Integer = 0
Private Sub InsertShipper(ByVal instance As Shipper)
    InsertShipperCalled += 1
    ' Call a Web service to perform an insert operation.
    InsertShipperService(shpr:=Nothing)
End Sub 

Public Shared UpdateShipperCalled As Integer = 0
Private Sub UpdateShipper(ByVal original As Shipper, ByVal current _
    As Shipper)
    UpdateShipperCalled += 1
    ' Call a Web service to update shipper.
    InsertShipperService(shpr:=Nothing)
End Sub 

Public Shared DeleteShipperCalled As Boolean 
Private Sub DeleteShipper(ByVal instance As Shipper)
    DeleteShipperCalled = True 
End Sub
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