TextBox的OnTextboxChanged事件里对Text重新赋值带中文， 导致崩溃

今天遇到一个超级bug， Textbox做了限制， 只能输入数字。 结果在搜狗输入法输入中文时导致崩溃， 出错信息如下：

未处理 System.InvalidOperationException
  Message=无法关闭撤消单元，因为不存在已打开的单元。

  Source=PresentationFramework
  StackTrace:
       在 MS.Internal.Documents.UndoManager.Close(IParentUndoUnit unit, UndoCloseAction closeAction)
       在 System.Windows.Documents.ImmComposition.CloseCompositionUndoUnit(UndoCloseAction undoCloseAction, ITextPointer compositionEnd)
       在 System.Windows.Documents.ImmComposition.UpdateCompositionText(FrameworkTextComposition composition, Int32 resultLength, Boolean includeResultText, ITextPointer& start, ITextPointer& end)
       在 System.Windows.Documents.ImmComposition.RaiseTextInputStartEvent(FrameworkTextComposition composition, Int32 resultLength, String compositionString)
       在 System.Windows.Documents.ImmComposition.OnWmImeChar(IntPtr wParam, Boolean& handled)
       在 System.Windows.Documents.ImmComposition.ImmCompositionFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       在 System.Windows.Interop.HwndSource.PublicHooksFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       在 MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       在 MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
       在 System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       在 MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
       在 System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
       在 MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       在 MS.Win32.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       在 MS.Win32.HwndSubclass.DefWndProcWrapper(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       在 MS.Win32.UnsafeNativeMethods.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       在 MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       在 MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
       在 System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
       在 System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
       在 System.Windows.Application.RunDispatcher(Object ignore)
       在 System.Windows.Application.RunInternal(Window window)
       在 System.Windows.Application.Run(Window window)
       在 System.Windows.Application.Run()
       在 LiveChainCHIS_Client.App.Main() 位置 D:项目LiveChainCHIS-ClientLiveChainCHIS-Clientobjx86DebugApp.g.cs:行号 0
       在 System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       在 System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       在 Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       在 System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
       在 System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       在 System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

补充说明一下 ，这个错误只出现于 数据绑定后的TextBox控件上。

解决办法：  设置Textbox的UndoLimit 为0就ok。 

可以参考： http://social.msdn.microsoft.com/Forums/zh-CN/wpfzhchs/thread/06b9f4a1-d4a5-456b-bedc-ffc8f8019445