Question

My base ViewModel, implemented a couple years ago, provides this extension method for running tasks while keeping the UI responsive*:

protected void Work(Action job)
{
    IsBusy = true;
    var stackTrace = new StackTrace();
    var task = Task.Factory.StartNew(job)
        .ContinueWith(failedTask => HandleException(failedTask, stackTrace), 
                      TaskContinuationOptions.OnlyOnFaulted)
        .ContinueWith(_ => { IsBusy = false; });
}

void HandleException(Task task, StackTrace stackTrace)
{
    Dispatcher.BeginInvoke(
        () => { throw new Exception(task.Exception.InnerException.ToString() + 
                                    stackTrace); });
}

IsBusy is a property observed from the UI to display a progress bar.

The idea behind HandleExceptions is that they are observed and then thrown in the UI thread, caught by a try/catch block in the Main() method and logged before displaying a friendly message to the user and safely closing the app. The stackTrace is passed so the log includes the caller information.

However, I recently started getting reports of the application crashing without logging, nor friendly messages, with this Windows dialog:

Looking at the Windows event log, we got this EventData:

Application: xxxxApplication.Loader.exe

Framework Version: v4.0.30319

Description: The process was terminated due to an unhandled exception.

Exception Info: System.AggregateException

Stack: at System.Threading.Tasks.TaskExceptionHolder.Finalize()

Am I doing anything wrong with ContinueWith()?

Is it possible for task exceptions to somehow stay unobserved?

---

_{*: I know about BackgroundWorker. This probably seemed like a better idea at the time, or had additional benefits.}

Answer 1

I would suggest you handle the AppDomain.CurrentDomain.UnhandledException
to either display your “friendly message” or (if this is not a solution for you) you could at least log the errors within this event and then see what the stacktrace is to determine where this “un-handled” task exception occurs.