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我在WPF中構建桌面應用程序，並且正在調試在UserControl的代碼後面的屬性更改方法中觸發的錯誤。當拋出異常時類變爲空
private void ErrorPanel_OnIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
{
FrameworkElementAutomationPeer.FromElement(this.ErrorMessage).RaiseAutomationEvent(AutomationEvents.AutomationFocusChanged);
}
當我在該方法的行上放置一個斷點時，我將鼠標懸停在此處並查看UserControl的正確類型。但是當我執行該行時，它會拋出一個對象引用null異常，並且這是null，這表明UserControl已被清除。
中的ErrorMessage的定義如下
<TextBlock x:Name="ErrorMessage"
Uid="7"
VerticalAlignment="Center"
Foreground="Red"
Text="Error message"
ToolTip="Error message"
AutomationProperties.HelpText="Error message"
AutomationProperties.Name="Error message"
Focusable="True"
FocusManager.IsFocusScope="True"
FocusVisualStyle="{StaticResource MyFocusVisual}"
FocusManager.FocusedElement="{Binding RelativeSource={RelativeSource Mode=Self}}"
/>
編輯
堆棧跟蹤爲
at FACTAsync.Views.MonitoringPlanListView.ErrorPanel_OnIsVisibleChanged(Object sender, DependencyPropertyChangedEventArgs e) in C:\TFS\CAMD Software Tools\FACT\FACTAsync\FACTAsync\Views\MonitoringPlanListView.xaml.cs:line 44
at System.Windows.UIElement.RaiseDependencyPropertyChanged(EventPrivateKey key, DependencyPropertyChangedEventArgs args)
at System.Windows.UIElement.OnIsVisibleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
at System.Windows.UIElement.UpdateIsVisibleCache()
at System.Windows.UIElement.OnVisibilityChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.FrameworkElement.OnPropertyChanged(DependencyPropertyChangedEventArgs e)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
at System.Windows.DependencyObject.InvalidateProperty(DependencyProperty dp, Boolean preserveCurrentValue)
at System.Windows.Data.BindingExpressionBase.Invalidate(Boolean isASubPropertyChange)
at System.Windows.Data.BindingExpression.TransferValue(Object newValue, Boolean isASubPropertyChange)
at System.Windows.Data.BindingExpression.ScheduleTransfer(Boolean isASubPropertyChange)
at MS.Internal.Data.ClrBindingWorker.NewValueAvailable(Boolean dependencySourcesChanged, Boolean initialValue, Boolean isASubPropertyChange)
at MS.Internal.Data.PropertyPathWorker.UpdateSourceValueState(Int32 k, ICollectionView collectionView, Object newValue, Boolean isASubPropertyChange)
at MS.Internal.Data.PropertyPathWorker.OnDependencyPropertyChanged(DependencyObject d, DependencyProperty dp, Boolean isASubPropertyChange)
at MS.Internal.Data.ClrBindingWorker.OnSourceInvalidation(DependencyObject d, DependencyProperty dp, Boolean isASubPropertyChange)
at System.Windows.Data.BindingExpression.HandlePropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
at System.Windows.Data.BindingExpressionBase.OnPropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
at System.Windows.Data.BindingExpression.OnPropertyInvalidation(DependencyObject d, DependencyPropertyChangedEventArgs args)
at System.Windows.DependentList.InvalidateDependents(DependencyObject source, DependencyPropertyChangedEventArgs sourceArgs)
at System.Windows.DependencyObject.NotifyPropertyChange(DependencyPropertyChangedEventArgs args)
at System.Windows.DependencyObject.UpdateEffectiveValue(EntryIndex entryIndex, DependencyProperty dp, PropertyMetadata metadata, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType)
at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, Boolean coerceWithCurrentValue, OperationType operationType, Boolean isInternal)
at System.Windows.DependencyObject.SetCurrentValueInternal(DependencyProperty dp, Object value)
at System.Windows.Controls.Primitives.ToggleButton.OnToggle()
at System.Windows.Controls.Primitives.ToggleButton.OnClick()
at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
at System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e)
at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
at System.Windows.UIElement.OnMouseUpThunk(Object sender, MouseButtonEventArgs e)
at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
at System.Windows.Input.InputManager.ProcessStagingArea()
at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
at System.Windows.Interop.HwndSource.InputFilterMessage(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 System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, 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.Application.RunDispatcher(Object ignore)
at System.Windows.Application.RunInternal(Window window)
at System.Windows.Application.Run(Window window)
at FACTAsync.Program.Main() in C:\TFS\CAMD Software Tools\FACT\FACTAsync\FACTAsync\Program.cs:line 60
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.ThreadHelper.ThreadStart_Context(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.Threading.ThreadHelper.ThreadStart()
評論是不適用於擴展討論;這個對話已經[轉移到聊天]（http://chat.stackoverflow.com/rooms/137221/discussion-on-question-by-kleineg-class-becomes-null-when-exception-is- thrown）。 –