HOW TO：管理物件內容中的資料並行存取 (Entity Framework)

本主題提供的範例將示範如何管理物件內容中的並行。 在這個主題中，您將在更新 SalesOrderHeader 物件的 Status 屬性時產生並處理 OptimisticConcurrencyException。 如需詳細資訊，請參閱儲存變更及管理並行存取 (Entity Framework)。

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 Entity Framework。 若要這樣做，請完成 HOW TO：手動設定 Entity Framework 專案和 HOW TO：手動定義模型和對應檔 (Entity Framework) 中的程序。

若要順利產生 OptimisticConcurrencyException，您必須在概念對應檔中修改 Status 屬性。

若要針對 Status 屬性啟用並行檢查

1. 開啟 AdventureWorks.csdl 檔案並且找出 SalesOrderHeader 實體的定義。

2. 尋找 Status 子元素，並加入下列屬性：

   ConcurrencyMode="fixed"
   

3. 將變更儲存到 AdventureWorks.csdl。

4. 在下列範例程式碼中，於 foreach 迴圈 (Loop) (Visual Basic 中的 For Each) 之後設定中斷點，然後以偵錯模式執行此應用程式。

5. 當執行作業中斷時，請使用 SQL Server Management Studio，針對 AdventureWorks 資料庫執行下列 Transact-SQL 命令：

   UPDATE Sales.SalesOrderHeader SET Status = 1 WHERE CreditCardApprovalCode IS NULL.
   

6. 重新啟動程式執行作業。

範例

在這個範例中，當您遵循上述程序之後，對 SalesOrderHeader 物件之 Status 屬性所做的變更就會導致 OptimisticConcurrencyException。

Using context As New AdventureWorksEntities()
    Try
        ' Perform an operation with a high-level of concurrency. 
        ' Change the status of all orders without an approval code. 
        Dim orders As ObjectQuery(Of SalesOrderHeader) = context.SalesOrderHeaders.Where("it.CreditCardApprovalCode IS NULL").Top("100")

        For Each order As SalesOrderHeader In orders
            ' Reset the order status to 4 = Rejected. 
            order.Status = 4
        Next
        Try
            ' Try to save changes, which may cause a conflict. 
            Dim num As Integer = context.SaveChanges()
            Console.WriteLine("No conflicts. " & num.ToString() & " updates saved.")
        Catch generatedExceptionName As OptimisticConcurrencyException
            ' Resolve the concurrency conflict by refreshing the 
            ' object context before re-saving changes. 
            context.Refresh(RefreshMode.ClientWins, orders)

            ' Save changes. 
            context.SaveChanges()
            Console.WriteLine("OptimisticConcurrencyException handled and changes saved")
        End Try

        For Each order As SalesOrderHeader In orders
            Console.WriteLine(("Order ID: " & order.SalesOrderID.ToString() & " Order status: ") + order.Status.ToString())
        Next
    Catch ex As UpdateException
        Console.WriteLine(ex.ToString())
    End Try
End Using

using (AdventureWorksEntities context =
    new AdventureWorksEntities())
{
    try
    {
        // Perform an operation with a high-level of concurrency.
        // Change the status of all orders without an approval code.
        ObjectQuery<SalesOrderHeader> orders =
            context.SalesOrderHeaders.Where(
            "it.CreditCardApprovalCode IS NULL").Top("100");

        foreach (SalesOrderHeader order in orders)
        {
            // Reset the order status to 4 = Rejected.
            order.Status = 4;
        }
        try
        {
            // Try to save changes, which may cause a conflict.
            int num = context.SaveChanges();
            Console.WriteLine("No conflicts. " +
                num.ToString() + " updates saved.");
        }
        catch (OptimisticConcurrencyException)
        {
            // Resolve the concurrency conflict by refreshing the 
            // object context before re-saving changes. 
            context.Refresh(RefreshMode.ClientWins, orders);

            // Save changes.
            context.SaveChanges();
            Console.WriteLine("OptimisticConcurrencyException "
            + "handled and changes saved");
        }

        foreach (SalesOrderHeader order in orders)
        {
            Console.WriteLine("Order ID: " + order.SalesOrderID.ToString()
                + " Order status: " + order.Status.ToString());
        }
    }
    catch (UpdateException ex)
    {
        Console.WriteLine(ex.ToString());
    }
}

另請參閱

概念

使用物件