NullReferenceException when hosting WPF in a System.AddIn

Question

We are developing a WPF application that uses the System.AddIn framework to host add-ins that display additional WPF content. Everything seems to be working fine, but overnight, the application threw the following NullReferenceException:

Message: Error : Object reference not set to an instance of an object.
StackTrace :   at System.Windows.Interop.HwndSource.CriticalTranslateAccelerator(MSG& msg, ModifierKeys modifiers)
   at System.AddIn.Pipeline.AddInHwndSourceWrapper.TranslateAccelerator(MSG msg, ModifierKeys modifiers)
   at MS.Internal.Controls.AddInHost.System.Windows.Interop.IKeyboardInputSink.TranslateAccelerator(MSG& msg, ModifierKeys modifiers)
   at System.Windows.Interop.HwndHost.OnKeyUp(KeyEventArgs e)
   at System.Windows.UIElement.OnKeyUpThunk(Object sender, KeyEventArgs e)
   at System.Windows.Input.KeyEventArgs.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(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.HwndKeyboardInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawKeyboardActions actions, Int32 scanCode, Boolean isExtendedKey, Boolean isSystemKey, Int32 virtualKey)
   at System.Windows.Interop.HwndKeyboardInputProvider.ProcessKeyAction(MSG& msg, Boolean& handled)
   at System.Windows.Interop.HwndSource.CriticalTranslateAccelerator(MSG& msg, ModifierKeys modifiers)
   at System.Windows.Interop.HwndSource.OnPreprocessMessage(Object param)
   at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Boolean isSingleParameter)
   at System.Windows.Threading.ExceptionWrapper.TryCatchWhen(Object source, Delegate callback, Object args, Boolean isSingleParameter, Delegate catchHandler)
   at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Boolean isSingleParameter)
   at System.Windows.Threading.Dispatcher.Invoke(DispatcherPriority priority, Delegate method, Object arg)
   at System.Windows.Interop.HwndSource.OnPreprocessMessageThunk(MSG& msg, Boolean& handled)
   at System.Windows.Interop.HwndSource.WeakEventPreprocessMessage.OnPreprocessMessage(MSG& msg, Boolean& handled)
   at System.Windows.Interop.ThreadMessageEventHandler.Invoke(MSG& msg, Boolean& handled)
   at System.Windows.Interop.ComponentDispatcherThread.RaiseThreadMessage(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.RunInternal(Window window)
   at System.Windows.Application.Run(Window window)
   at System.Windows.Application.Run()

As you can see, none of our code is in the stack trace, so I have no place to fix anything. Anybody have any ideas about possible workarounds?

Thanks for the help!

Answer 1

I believe the first thing you need to look at is changing a setting in Visual Studio. I had a number of issues with this and had to go in an manually tell Visual Studio to break on that NullReferenceException. Once you make this adjustment the IDE will actually throw the error and you can debug your code. From the VS 2008 help:

To break execution when an exception is thrown On the Debug menu, click Exceptions.

In the Exceptions dialog box, select Thrown for an entire category of exceptions, for example, Common Language Runtime Exceptions.

-or-

Expand the node for a category of exceptions, for example, Common Language Runtime Exceptions, and select Thrown for a specific exception within that category.

Answer 2

In addition the above answer by Steve Massing it will likely help to disable the "Just My Code" option, enable the "Enable source server support" option and setup your debugging symbols in order to break into and debug the framework code,

It's easier than it sounds, follow these 3 step directions

Red line Unchecked, Green line Checked and setup symbols like below

<sub>(source: gangleri.net)

(source: gangleri.net)</sub>