Rethrowing exceptions in tasks c# without wait

Question

I have a GUI application, in which I want to run something in a task, so it will not hold the UI. I want un unhandled exception in the task to be propogated to the application level exception handler. However:

If I just throw an exception in the task it will not reach app level exceptions unless I use wait/await
Async/Await - I call the method from a UI constructor, so I can't use async/await there, since I need to continue with the consturction. I just want to run the task and forget.

I was thinking about using dispatcher.invoke, what do you think?

public MainWindow()
{
        InitializeComponent();

        MyMethodAsync();

        InitializeA();
        IntiializeB();
}

private void MyMethodAsync()
{
     Task t = Task.Run(() =>
     {
          //Do some stuff
          throw new Exception("Throwing some unexpected exception");
     }).ContinueWith(MyContinueWith);
}

private void MyContinueWith(Task task)
{
    if (task.IsFaulted && task.Exception != null)
    {
         dispatcher.BeginInvoke(new Action(() =>
         {
            throw task.Exception;
         }), null);
    }
}

By convention methods that have a suffix of "Async" are normally methods which return a task that has already been started. With the async/await keywords this is handled somewhat automatically to the caller it doesn't really matter the implementation. — jpierson
@skippy: await doesn't block your UI. It sounds like await has exactly the behavior you're wanting. — Stephen Cleary
Why are you using Task.ContinueWith? It really isn't recommended, you should use C#5 await instead. The debugging potential alone makes it insane to use Task.continueWith. — Aron
Thanks for the quick answers. The reason that I don't want to use await, is that I want to use MyMethodAsync, in a constructor that run on UI thread. So I don't to wait and hold the construction of my window. (I cannot make a constructor to be async) — skippy

Yuval Itzchakov Yuval Itzchakov · Accepted Answer · 2015-10-15T10:36:21

Two ways I can think of. First, is register to TaskScheduler.UnobservedTaskException event and log whatever you need there:

private void MyMethodAsync()
{
    // Note you should probably register only once, so this may not fit here.
    TaskScheduler.UnobservedTaskException += (s, e) => GlobalLogger.Log(e);
    Task t = Task.Run(() =>
    {
        // Do some staff
    }).ContinueWith(MyContinueWith);
}

The better option which for some reason you don't want to use, is to actually await the operation and wrap it in a try-catch:

private async Task MyMethodAsync()
{
    try
    {
       await Task.Run(() =>
       {
          // Do some staff
       });
       InvokeContinuation();
    }
    catch (Exception e)
    {
        // Log.
    }
}

Rethrowing exceptions in tasks c# without wait

2 Answers