Question

I’m building a WPF application using the SimpleMVVM framework and I’m having trouble catching exceptions. I use the MessageBus of SimpleMVVM to send a message to another viewmodel. This all works fine, but I noticed that exceptions raised in the code executed by the messagebus get suppressed. Here’s what I’ve got so far:

My MainWindow contains a button that fires a TempCommand on the MainWindowViewModel. This command in turn calls the Test method (shown below), which sends out a notification message using the MessageBus of SimpleMVVM.

private void Temp()
{
    SendMessage("Temp", new NotificationEventArgs());
}

My MainWindow also contains a Frame with content. The ViewModel of this content, CustomerViewModel, has registered to receive these notifications in its constructor:

public CustomerDetailsViewModel(ICustomerServiceAgent agent)
{
    RegisterToReceiveMessages("Temp", Temp);
}

Where the Temp method simply throws an exception:

private void Temp(object sender, NotificationEventArgs args)
{
    throw new NotImplementedException("Somewhere, something horrible happened");
}

When I debug the application, I clearly see the Temp method being called and the exception being raised. But for some reason, that’s all. The application is unaffected and my exception trapping code is unaware of the exception.

I trap exceptions in two ways. The first is by handling the event on the Dispatcher:

<Application x:Class="MyApp"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             StartupUri="MainWindow.xaml"
             DispatcherUnhandledException="App_DispatcherUnhandledException">

Where the code-behind looks like:

private void App_DispatcherUnhandledException(object sender, DispatcherUnhandledExceptionEventArgs e)
{
    Log("Exception: " + e.Exception.Message);
    e.Handled = true;
}
public static void Log(string message)
{
    File.AppendAllText(@"D:\Temp\log.txt", "[" + DateTime.Now.ToString("F") + "] [" + System.Threading.Thread.CurrentThread.ManagedThreadId.ToString() + "] " + message + Environment.NewLine);
}

This code catches some exceptions, but not all. I found out that WPF suppresses databinding exceptions by default. Because my ViewModels are bounded through the DataContext property on my view, I thought this was the problem. I found this article, which defines a TraceListener that uses the PresentationTraceSources class. Databinding exceptions now get caught, but… Not the exceptions thrown in the code executed through the MessageBus.

I’ve created a solution demonstrating this behavior, it can be downloaded here.

And this is where I’m stuck. What am I missing? How do I catch these exceptions?

Big thanks in advance.

JP

Answer 1

I think it is a bug or problem with the implementation of the MessageBus in SimpleMVVM.

Cause multiple subscribers can subscribe to a token, the current implementation ensures that each subscribed method gets called even when one registered method throws an exception. In this case the exception is catched and written out to the Console.

The method that is responsible to call a subscribed method is SafeNotify

private void SafeNotify(Action method, bool post) {
  try {
    // Fire the event on the UI thread
    if (post){
      if (Dispatcher.CheckAccess()){
        method();
      }
      else{
        Dispatcher.BeginInvoke(method);
      }
    }
    // Fire event on a ThreadPool thread
    else{
      ThreadPool.QueueUserWorkItem(o => method(), null);
    }
  }
  catch (Exception ex){
    // If there's an exception write it to the Output window
    Debug.WriteLine(ex.ToString());
  }
}

When the method call gets queued in the ThreadPool, you have no chance to handle the thrown exception. See also this post for further information.

The only option you have is to ensure that the code of your own registered methods is always surrounded by a try-catch-block.