How to: Execute Business Logic During Association Changes

This topic shows how to execute business logic when an association between entities changes.

The example in this topic is based on the Adventure Works Sales Model. To run the code in this topic, you must have already added the Adventure Works Sales Model to your project and configured your project to use the Entity Framework. For more information, see How to: Use the Entity Data Model Wizard (Entity Framework) or How to: Manually Configure an Entity Framework Project and How to: Manually Define an Entity Data Model (Entity Framework). 

You must also add the following using statement (Imports in Visual Basic) to your code:

Imports System.ComponentModel

using System.ComponentModel;

Example

This example extends the example in the How to: Use an EntityReference to Change Relationships Between Objects topic. This example shows how to check the order status when the shipping address changes by handling the AssociationChanged event on the EntityReference for the Address object that represents the shipping address. If the order status is greater than 3, the order cannot be changed and an exception is raised. The delegate is defined in the constructor for the SalesOrderHeader partial class, and the handler for this event is also implemented in this partial class. This ensures that the order status is checked whenever the shipping address is changed for an order.

You can also call the OnPropertyChanging and OnPropertyChanged methods in this event to raise the PropertyChanging and PropertyChanged events, respectively. These events notify client controls of the association changes.

To validate changes at the other end of the SalesOrderHeader-Address relationship, a similar technique could be used to register the AssociationChanged event on the EntityCollection of SalesOrderHeader objects that are related to a shipping address.

Partial Public Class SalesOrderHeader
    ' SalesOrderHeader default constructor. 
    Public Sub New()
        ' Register the handler for changes to the 
        ' shipping address (Address1) reference. 
        AddHandler Me.AddressReference.AssociationChanged, AddressOf ShippingAddress_Changed
    End Sub

    ' AssociationChanged handler for the relationship 
    ' between the order and the shipping address. 
    Private Sub ShippingAddress_Changed(ByVal sender As Object, ByVal e As CollectionChangeEventArgs)
        ' Check for a related reference being removed. 
        If e.Action = CollectionChangeAction.Remove Then
            ' Check the order status and raise an exception if 
            ' the order can no longer be changed. 
            If Me.Status > 3 Then
                Throw New InvalidOperationException("The shipping address cannot " & _
                                                    "be changed because the order has either " & _
                                                    "already been shipped or has been cancelled.")
            End If
            ' Call the OnPropertyChanging method to raise the PropertyChanging event. 
            ' This event notifies client controls that the association is changing. 
            Me.OnPropertyChanging("Address1")
        ElseIf e.Action = CollectionChangeAction.Add Then
            ' Call the OnPropertyChanged method to raise the PropertyChanged event. 
            ' This event notifies client controls that the association has changed. 
            Me.OnPropertyChanged("Address1")
        End If
    End Sub
End Class

public partial class SalesOrderHeader
{
    // SalesOrderHeader default constructor.
    public SalesOrderHeader()
    {
        // Register the handler for changes to the 
        // shipping address (Address1) reference.
        this.AddressReference.AssociationChanged
            += new CollectionChangeEventHandler(ShippingAddress_Changed);
    }

    // AssociationChanged handler for the relationship 
    // between the order and the shipping address.
    private void ShippingAddress_Changed(object sender,
        CollectionChangeEventArgs e)
    {
        // Check for a related reference being removed. 
        if (e.Action == CollectionChangeAction.Remove)
        {
            // Check the order status and raise an exception if 
            // the order can no longer be changed.
            if (this.Status > 3)
            {
                throw new InvalidOperationException(
                    "The shipping address cannot "
                + "be changed because the order has either "
                + "already been shipped or has been cancelled.");
            }
            // Call the OnPropertyChanging method to raise the PropertyChanging event.
            // This event notifies client controls that the association is changing.
            this.OnPropertyChanging("Address1");
        }
        else if (e.Action == CollectionChangeAction.Add)
        {
            // Call the OnPropertyChanged method to raise the PropertyChanged event.
            // This event notifies client controls that the association has changed.
            this.OnPropertyChanged("Address1");
        }
    }
}

See Also

Tasks

How to: Execute Business Logic When the Object State Changes
How to: Execute Business Logic During Scalar Property Changes
How to: Execute Business Logic When Saving Changes