WP7 LINQ Delete不工作-获取NullReferenceException

我正在开发一个WP7应用程序，我需要使用DeleteOnSubmit方法删除一行，但我一直收到一个NullReferenceException错误。我找不到问题所在

public void HardDeleteOrder(int deleteOrderId)
{

    var oResult = from o in App.orderDataContext.orders
              where o.OrderId == deleteOrderId
              select o;


    foreach (var oRow in oResult)
    {
       App.orderDataContext.Orders.DeleteOnSubmit(oRow);
    }
    App.orderDataContext.SubmitChanges();
}

当我运行此命令时，代码在方法的结束大括号上崩溃，出现异常消息NullReferenceException未处理

以下是堆栈跟踪：

System.NullReferenceException was unhandled
Message=NullReferenceException
StackTrace:
   at System.Data.Linq.Mapping.MetaAccessor`2.SetBoxedValue(Object& instance, Object value)
   at System.Data.Linq.ChangeProcessor.ClearForeignKeysHelper(MetaAssociation assoc, Object trackedInstance)
   at System.Data.Linq.ChangeProcessor.ClearForeignKeyReferences(TrackedObject to)
   at System.Data.Linq.ChangeProcessor.PostProcessUpdates(List`1 insertedItems, List`1 deletedItems)
   at System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode)
   at System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode)
   at System.Data.Linq.DataContext.SubmitChanges()
   at orders.viewmodels.OrderViewModel.HardDeleteOrder(Int32 deleteOrderId)
   at orders.OrderView.RemoveOrderFromDatabase()
   at orders.OrderView.RemoveOrder()
   at orders.OrderView.detailsBarCancel_Click(Object sender, EventArgs e)
   at orders.App.detailsBarCancel_Click(Object sender, EventArgs e)
   at Microsoft.Phone.Shell.ApplicationBarItemContainer.FireEventHandler(EventHandler handler, Object sender, EventArgs args)
   at Microsoft.Phone.Shell.ApplicationBarIconButton.ClickEvent()
   at Microsoft.Phone.Shell.ApplicationBarIconButtonContainer.ClickEvent()
   at Microsoft.Phone.Shell.ApplicationBar.OnCommand(UInt32 idCommand)
   at Microsoft.Phone.Shell.Interop.NativeCallbackInteropWrapper.OnCommand(UInt32 idCommand)

---

这里缺少什么？

这主要是由于外键约束。 根据

LINQ to SQL不支持或识别级联删除操作。如果要删除表中具有约束的行，必须完成以下任一任务：

Set the ON DELETE CASCADE rule in the foreign-key constraint in the database.

Use your own code to first delete the child objects that prevent the parent object from being deleted.