HOW TO：變更物件之間的關聯性 (Entity Framework)
本主題示範如何使用 EntityReference 物件來變更物件內容中兩個物件之間的關聯性。 當呼叫 SaveChanges 方法時，關聯性變更會當做關聯資料表內外部索引鍵的變更保存在資料庫中。 本主題也會示範如何處理 AssociationChanged 事件。
本主題的範例是根據 Adventure Works Sales Model。 若要執行此範例中的程式碼，您必須已經將 AdventureWorks Sales Model 加入到專案中，並設定您的專案使用 實體架構。 若要這樣做，請完成 HOW TO：手動設定 Entity Framework 專案和 HOW TO：以手動方式定義 Entity Data Model (Entity Framework) 中的程序。 您也必須將以下 using 陳述式 (Visual Basic 中的 Imports) 加入到程式碼：
Imports System.ComponentModel
using System.ComponentModel;
範例
這則範例將示範如何使用 EntityReference 物件來變更 SalesOrderHeader 物件與代表訂單送貨地址之相關 Address 物件之間的關聯性。
'Define the order and new address IDs.
Dim orderId As Integer = 43669
Dim newAddressId As Integer = 26
Using context As New AdventureWorksEntities()
Try
' Get the billing address to change to.
Dim newAddress As Address = context.Address _
.Where("it.AddressID = @addressId", _
New ObjectParameter("addressId", newAddressId)) _
.First()
' Get the order being changed.
Dim order As SalesOrderHeader = context.SalesOrderHeader _
.Where("it.SalesOrderID = @orderId", _
New ObjectParameter("orderId", orderId)).First()
' Load the current billing address.
If Not order.Address1Reference.IsLoaded Then
order.Address1Reference.Load()
End If
' Write the current billing street address.
Console.WriteLine("Current street: " _
+ order.Address1.AddressLine1)
' Change the billing address.
If Not order.Address1.Equals(newAddress) Then
order.Address1 = newAddress
' Write the changed billing street address.
Console.WriteLine("Changed street: " _
+ order.Address1.AddressLine1)
End If
' If the address change succeeds, save the changes.
context.SaveChanges()
' Write the current billing street address.
Console.WriteLine("Current street: " _
+ order.Address1.AddressLine1)
Catch ex As ApplicationException
' Handle the exception raised in the ShippingAddress_Changed
' handler when the status of the order prevents the
' shipping address from being changed. Don't retry because
' the relationship is in an inconsistent state and calling
' SaveChanges() will result in an UpdateException.
Console.WriteLine(ex.ToString())
Catch ex As InvalidOperationException
Console.WriteLine(ex.ToString())
End Try
End Using
// Define the order and new address IDs.
int orderId = 43669;
int newAddressId = 26;
using (AdventureWorksEntities context
= new AdventureWorksEntities())
{
try
{
// Get the billing address to change to.
Address newAddress = context.Address
.Where("it.AddressID = @addressId",
new ObjectParameter("addressId", newAddressId))
.First();
// Get the order being changed.
SalesOrderHeader order = context.SalesOrderHeader
.Where("it.SalesOrderID = @orderId",
new ObjectParameter("orderId", orderId)).First();
// Load the current billing address.
if (!order.Address1Reference.IsLoaded)
{
order.Address1Reference.Load();
}
// Write the current billing street address.
Console.WriteLine("Current street: "
+ order.Address1.AddressLine1);
// Change the billing address.
if (!order.Address1.Equals(newAddress))
{
order.Address1 = newAddress;
// Write the changed billing street address.
Console.WriteLine("Changed street: "
+ order.Address1.AddressLine1);
}
// If the address change succeeds, save the changes.
context.SaveChanges();
// Write the current billing street address.
Console.WriteLine("Current street: "
+ order.Address1.AddressLine1);
}
catch (ApplicationException ex)
{
// Handle the exception raised in the ShippingAddress_Changed
// handler when the status of the order prevents the
// shipping address from being changed. Don't retry because
// the relationship is in an inconsistent state and calling
// SaveChanges() will result in an UpdateException.
Console.WriteLine(ex.ToString());
}
catch (InvalidOperationException ex)
{
Console.WriteLine(ex.ToString());
}
}
此範例會擴充之前的範例，並示範如何在變更送貨地址時檢查訂單狀態，其方式是針對代表送貨地址的 Address 物件來處理 EntityReference 上的 AssociationChanged 事件。 如果訂單狀態大於 3，就無法變更訂單，而且會引發例外狀況。 此委派會定義在 SalesOrderHeader 部分類別的建構函式 (Constructor) 中，而且此事件的處理常式也會在這個部分類別中實作。 如此可確保每當訂單的送貨地址變更時，都會檢查訂單狀態。
若要驗證 SalesOrderHeader-Address 關聯性另一端的變更，可使用類似的技巧在與送貨地址有關之 SalesOrderHeader 物件的 EntityCollection 上註冊 AssociationChanged 事件。
Partial Public Class SalesOrderHeader
' SalesOrderHeader default constructor.
Public Sub New()
' Register the handler for changes to the
' shipping address (Address1) reference.
AddHandler Me.Address1Reference.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 ApplicationException( _
"The shipping address cannot " _
+ "be changed because the order has either " _
+ "already been shipped or has been cancelled.")
End If
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.Address1Reference.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 ApplicationException(
"The shipping address cannot "
+ "be changed because the order has either "
+ "already been shipped or has been cancelled.");
}
}
}
}