为什么我有时会在 BeginInvoke 块中得到 NullReferenceException？

Question

所以在我的 XXX.OnPropertyChanged() 方法中我有：

public class XXX : IProperyNotifyChanged {
   Control itsCtrl;
   ...

   public void Init(Control ctrl) {
       itsCtrl = ctrl;
   }

   public void OnPropertyChanged(string propertyName) {
    if (PropertyChanged != null) {
        if (itsCtrl.InvokeRequired) {
            itsCtrl.BeginInvoke(() => {
              PropertyChanged(this, propertyName);
             });
        } else {
            PropertyChanged(this, propertyName);
         }
      }
   }
}

我认为这会抛出以下异常（很少发生，但现在更常发生）：

System.Reflection.TargetInvocationException was unhandled
  HResult=-2146232828
  Message=Exception has been thrown by the target of an invocation.
  Source=mscorlib
  StackTrace:
       at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
       at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
       at System.Delegate.DynamicInvokeImpl(Object[] args)
       at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)
       at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)
       at System.Windows.Forms.Control.InvokeMarshaledCallbacks()
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.Form.WndProc(Message& m)
       at DevExpress.XtraEditors.XtraForm.WndProc(Message& msg)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at DevExpress.Utils.Win.Hook.ControlWndHook.CallWindowProc(IntPtr pPrevProc, IntPtr hWnd, Int32 message, IntPtr wParam, IntPtr lParam)
       at DevExpress.Utils.Win.Hook.ControlWndHook.WindowProc(IntPtr hWnd, Int32 message, IntPtr wParam, IntPtr lParam)
       at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
       at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at Client.Program.Main() in C:\Client\Program.cs:line 18
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 
       HResult=-2147467261
       Message=Object reference not set to an instance of an object.
       Source=XXX
       StackTrace:
            at XXX.<>c__DisplayClass442_0.<OnPropertyChanged>b__0()
       InnerException:

我只是在想。发生这种情况是因为我在调用 BeginInvoke 之前没有像 this 和 propertyName 那样正确地复制变量吗？或者是别的什么？这种情况很少发生，以至于我不确定如何重现它，而且我真的无法从堆栈跟踪中获得太多信息。你会如何解决这个问题？

Answer 1

我强烈建议使用 C# 6.0 带来的空条件运算符，如果可以的话：

itsCtrl.InvokeRequired(...)   should be     itsCtrl?.InvokeRequired(...)
itsCtrl.BeginInvoke(...)      should be     itsCtrl?.BeginInvoke(...)

与您所相信的不同，在加载表单时，您的控件可能 null，因此您从竞争条件中获得异常。

您应该对 PropertyChanged 调用执行相同的操作：

PropertyChanged(...) should be  PropertyChanged?.Invoke(...)

这是线程安全的，可以避免您的检查 if (PropertyChanged != null) 由于其他线程更改而不再正确的情况。

Answer 2

I was just thinking. Is this happening because I am not copying variables properly like this and propertyName before calling BeginInvoke?

this 本质上总是在栈上，不能赋值给别的东西，所以不能在方法内设置为null。 propertyName 是本地的，所以那里不能比赛。

PropertyChanged虽然不是本地的，但是每次都获取。当你这样做时：

if (PropertyChanged != null)
{
  PropertyChanged.BeginInvoke(…);
}

它的作用如下：

PropertyChangedEventHandler local1 = PropertyChanged; // Get value from property;
if (local1 != null)
{
  PropertyChangedEventHandler local2 = PropertyChanged; // Get value from property;
  local2.BeginInvoke(…);
}

同时 PropertyChanged 有机会被设置为 null。那是您想复制的内容：

var propChanged = PropertyChanged;
if (propChanged != null)
{
  propChanged.BeginInvoke(…);
}

现在 propChanged 在整个方法的持续时间内要么为空，要么不为空，比赛结束。

的确如此：

PropertyChanged?.BeginInvoke(…);

为什么我有时会在 BeginInvoke 块中得到 NullReferenceException？

Why do I sometimes get a NullReferenceException in the BeginInvoke block?

.net

c#

invoke

begininvoke