如何：在对象上下文中管理数据并发性（实体框架）
本主题提供如何在对象上下文中管理并发性的示例。在本主题中，您将在更新 SalesOrderHeader 对象的 Status 属性时生成和处理 OptimisticConcurrencyException。有关更多信息，请参见保存更改和管理并发（实体框架）。
本主题中的示例基于 Adventure Works 销售模型。若要运行本示例中的代码，必须已将 AdventureWorks 销售模型添加到您的项目中，并将项目配置为使用 Entity Framework。为此，请完成如何：手动配置实体框架项目和如何：手动定义实体数据模型（实体框架） 中的过程。
为了成功地生成 OptimisticConcurrencyException，必须在概念映射文件中修改 Status 属性。
对 Status 属性启用并发检查
打开 AdventureWorks.csdl 文件并找到 SalesOrderHeader 实体的定义。
查找子 Status 元素，然后添加以下属性：
ConcurrencyMode="fixed"保存对 AdventureWorks.csdl 的更改。
在以下示例代码中，在 foreach 循环（在 Visual Basic 中为 For Each）后设置断点，并在调试模式下运行应用程序。
当执行断开后，使用 SQL Server Management Studio 以针对 AdventureWorks 数据库执行以下 Transact-SQL 命令：
UPDATE Sales.SalesOrderHeader SET Status = 1 WHERE CreditCardApprovalCode IS NULL.重新启动程序执行。
示例
在本示例中，当执行上述过程后，对 SalesOrderHeader 对象的 Status 属性所做的更改将导致出现 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.SalesOrderHeader.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 ex 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.SalesOrderHeader.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());
}
}