PresentationFramework.dll的NullReferenceException [英] NullReferenceException from PresentationFramework.dll

问题描述

我正在与一个奇怪的NullReferenceException进行战斗，这显然是从 GetNameCore（）函数的 ItemAutomationPeer 类。

I'm fighting with a strange NullReferenceException that is apparently fired from the GetNameCore() function of the ItemAutomationPeer class.

异常的细节如下，但真正有趣的方面是，在我的开发机器上运行Windows 7或其他已经测试的Windows 7计算机。它只发生在我的Windows 8 Pro测试机器上。

The details of the exception are below but the really intriguing aspect is that it doesn't occur on my development machine running Windows 7, or other Windows 7 computers we have tested with. It only happens on my Windows 8 Pro test machine.

当尝试在WPF DataGrid控件中编辑单元格时，显然会抛出异常。

The exception is thrown apparently when attempting to edit a cell in a WPF DataGrid control.

我一直在努力追踪它一整天没有成功。我尝试使用Visual Studio进行远程调试，并逐步执行代码，但是没有任何用户代码似乎触发了异常。这显然是由PresentationFramework.Dll内的一系列事件执行的，异常只是通过AppDomain冒泡起来，最终会崩溃应用程序。

I've been trying to track it down all day without success. I tried debugging the process remotely using Visual Studio and stepping through the code but none of the user code seems to be triggering the exception. It's obviously executed by a sequence of events inside of the PresentationFramework.Dll and the exception just bubbles up through the AppDomain and eventually crashes the application.

如果有人能想到任何东西这可能是导致这种情况，或者一种解决它的方法它真的有帮助。

If anybody can think of anything that might be causing this, or a way to fix it it would really help.

Exception Type:         System.NullReferenceException
Exception Message:  Object reference not set to an instance of an object.
Method Information: System.String GetNameCore()
Exception Source:   PresentationFramework

Stack Trace
  at System.Windows.Automation.Peers.ItemAutomationPeer.GetNameCore()
  at System.Windows.Automation.Peers.AutomationPeer.UpdateSubtree()
  at System.Windows.Automation.Peers.AutomationPeer.UpdateSubtree()
  at System.Windows.Automation.Peers.AutomationPeer.UpdateSubtree()
  at System.Windows.ContextLayoutManager.fireAutomationEvents()
  at System.Windows.ContextLayoutManager.UpdateLayout()
  at System.Windows.ContextLayoutManager.UpdateLayoutCallback(Object arg)
  at System.Windows.Media.MediaContext.InvokeOnRenderCallback.DoWork()
  at System.Windows.Media.MediaContext.FireInvokeOnRenderCallbacks()
  at System.Windows.Media.MediaContext.RenderMessageHandlerCore(Object    resizedCompositionTarget)
  at System.Windows.Media.MediaContext.AnimatedRenderMessageHandler(Object resizedCompositionTarget)
  at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
  at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
  at System.Windows.Threading.DispatcherOperation.InvokeImpl()
  at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state)
  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.Threading.DispatcherOperation.Invoke()
  at System.Windows.Threading.Dispatcher.ProcessQueue()
  at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
  at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
  at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
  at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
  at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
  at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
  at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
  at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
  at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
  at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
  at System.Windows.Threading.Dispatcher.Run()
  at System.Windows.Application.RunDispatcher(Object ignore)
  at System.Windows.Application.RunInternal(Window window)
  at System.Windows.Application.Run(Window window)
  at System.Windows.Application.Run()

推荐答案

经过与远程调试器的大量连接，几乎无法在线搜索，我能够将这个问题跟踪到几个错误的行为 ItemAutomationPeer 实例。

After a lot of back-and-forth with the remote debugger, and nearly fruitless searches online I was able to track down the issue to a couple of miss-behaving ItemAutomationPeer instances.

当我碰到了这个问题，我对于UI自动化有了这么多的知识，以及它在WPF框架中的支持。事实上，当自动化程序由于某种原因让我想到COM互操作时，所以我追赶了一段错误的问题。如果您正在阅读这篇文章，并且不知道UI Automation可能在此处和这里可能会给你一个想法，就像UI自动化意味着什么WPF的上下文。

When I ran into this issue I had zero knowledge about UI Automation and how it is supported in the WPF framework. In fact, when AutomationPeer had me thinking of COM interop for some reason so I chased the wrong issue for a while. If you are reading this and don't know what UI Automation is perhaps starting here and here might give you an idea as far as what UI Automation means in the context of WPF.

在我的情况下，事实证明，应用程序在Windows 8测试机上崩溃的原因，但在我的开发机器上工作正常（以及其部署的无数其他计算机）是Windows 8机器具有某种UI辅助功能应用程序（或其他UI Automation客户端）运行。一旦我在Windows 7开发机器上开始讲解者应用程序，我就能够让应用程序崩溃一样。

In my case, it turns out that the reason why the application was crashing on the Windows 8 test machine yet it was working fine on my development machine (and countless other computers that it had been deployed to) was that the Windows 8 machine had some sort of UI accessibility application (or some other UI Automation client) running. As soon as I started the Narrator application on my Windows 7 development machine I was able to get the app to crash just the same..

一旦我了解根本问题，我仍然无法进一步调试，以确定哪个控件是导致问题，但更多的在线阅读似乎指向了自定义控件的一般方向，所以我开始一个消除过程来确定哪些自定义WPF控件是有罪的。我发现两个自定义控件 - 一个扩展了一个DataGrid，另一个扩展了一个ListBox。

Once I understood the root problem, I still was unable to further debug this to find out exactly which control was causing the problem but more reading online seemed to point in the general direction of custom controls and so I started a process of elimination to determine which custom WPF controls were guilty. I found two custom controls - one that extended a DataGrid, and another that extended a ListBox.

最后，解决我的问题的方法是创建自定义类扩展 ItemsControlAutomationPeer 基类，并将其作为自动化对等体在覆盖 OnCreateAutomationPeer 方法的每个具有问题的自定义控件中。

Finally, the solution to the problem in my case was to create custom classes that extend the ItemsControlAutomationPeer base class and to provide those as automation peers on each of the custom controls that had issues by overriding the OnCreateAutomationPeer method.

protected override AutomationPeer OnCreateAutomationPeer()
{
    return new ControlSpecificCustomAutomationPeer(this);
}

ControlSpecificCustomAutomationPeer类可能至少看起来像这样： p>

Where the ControlSpecificCustomAutomationPeer class might looks something like this at the very least:

public class ControlSpecificCustomAutomationPeer
    : ItemsControlAutomationPeer
{
    public ControlSpecificCustomAutomationPeer(ItemsControl owner)
        : base(owner)
    {
    }

    protected override string GetNameCore()
    {
        return "";                         // return something meaningful here..
    }

    protected override ItemAutomationPeer CreateItemAutomationPeer(object item)
    {
        return new CustomDummyItemAutomationPeer(item, this);
    }             
}

public class CustomDummyItemAutomationPeer
    : System.Windows.Automation.Peers.ItemAutomationPeer
{
    public CustomDummyItemAutomationPeer(object item, ItemsControlAutomationPeer itemsControlAutomationPeer)
        : base(item, itemsControlAutomationPeer)
    {
    }

    protected override string GetNameCore()
    {
        if (Item == null)
            return "";            

        return Item.ToString() ?? "";
    }

    protected override AutomationControlType GetAutomationControlTypeCore()
    {
        return System.Windows.Automation.Peers.AutomationControlType.Text;
    }

    protected override string GetClassNameCore()
    {
        return "Dummy";
    }
}

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