Bug 16676 - Tasks don't handle OperationCancelledException correctly
Summary: Tasks don't handle OperationCancelledException correctly
Alias: None
Product: Android
Classification: Xamarin
Component: BCL Class Libraries ()
Version: 4.10.1
Hardware: PC Windows
: --- normal
Target Milestone: ---
Assignee: Jonathan Pryor
Depends on:
Reported: 2013-12-09 10:22 UTC by Grigory (Playtika)
Modified: 2015-07-13 15:53 UTC (History)
2 users (show)

Is this bug a regression?: ---
Last known good build:

Crash (203.34 KB, image/png)
2013-12-10 02:36 UTC, Grigory (Playtika)

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Description Grigory (Playtika) 2013-12-09 10:22:18 UTC
	await ThrowMyException();
        // Unhandled TaskCancelledException here
	Toast.MakeText(this,"Wohoo", ToastLength.Long).Show();
	// We don't get here. WTF?!



public async Task ThrowMyException()
	throw new MyException ();

public class MyException : System.OperationCanceledException

Im not sure if this is a Mono bug or a Xamarin.Android. But anyway this is quite buggy logic
Comment 1 Grigory (Playtika) 2013-12-09 10:27:47 UTC
And please review all your code related to tasks and Aggregate Exceptions.
I have a project for Android and Windows (dekstop Windows Forms app on .NET 4.5)
On Android i see huge amount of unhandled Aggregate exceptions. I've checked every line of code. I suppose that situation there is like this. Some exceptions are simply not caught. I don't know why but this is really a BLOCKER issue in Xamarin. All those cool contact and GPS apis have zero value when such issues exist.
Comment 2 Jonathan Pryor 2013-12-09 14:03:45 UTC
I'm going to need a more complete test case. The following code works for me:

  using System;
  using System.Threading.Tasks;
  using Android.App;
  using Android.Widget;
  using Android.OS;

  namespace Scratch.Bxc16676 {
    [Activity (Label = "Scratch.Bxc16676", MainLauncher = true)]
    public class MainActivity : Activity {
      int count = 1;

      protected override async void OnCreate (Bundle bundle)
        base.OnCreate (bundle);

        try {
          await ThrowMyException ();
        } catch (MyException e) {
          Console.WriteLine ("Caught exception: {0}", e);

        // Set our view from the "main" layout resource
        SetContentView (Resource.Layout.Main);

        // Get our button from the layout resource,
        // and attach an event to it
        Button button = FindViewById<Button> (Resource.Id.myButton);
        button.Click += delegate {
          button.Text = string.Format ("{0} clicks!", count++);

      public static async Task ThrowMyException ()
        throw new MyException ();

    class MyException : System.OperationCanceledException {

The above writes the following to `adb logcat`:

> I/mono-stdout( 3566): Caught exception: Scratch.Bxc16676.MyException: The operation was canceled.
> I/mono-stdout( 3566):   at Scratch.Bxc16676.MainActivity+<ThrowMyException>c__async1.MoveNext ()
> I/mono-stdout( 3566):   at System.Runtime.CompilerServices.AsyncTaskMethodBuilder.Start[<ThrowMyException>c__async1] (Scratch.Bxc16676.<ThrowMyException>c__async1& stateMachine)
> I/mono-stdout( 3566):   at Scratch.Bxc16676.MainActivity.ThrowMyException ()
> I/mono-stdout( 3566):   at Scratch.Bxc16676.MainActivity+<OnCreate>c__async0.MoveNext () [0x00061]

Which Xamarin.Android version are you using?
Comment 3 Grigory (Playtika) 2013-12-09 17:00:10 UTC
Version: 4.10.1 (Enterprise Edition)

Stable branch.

I'll attach repro project tomorrow. App crashes for me with same code.
Comment 4 Grigory (Playtika) 2013-12-10 02:36:01 UTC
Created attachment 5626 [details]
Comment 5 Grigory (Playtika) 2013-12-10 03:14:36 UTC
I've ran your code and it has crashed.
Comment 6 Jonathan Pryor 2013-12-10 12:31:21 UTC
I suspect this may be a dupe of Bug #14824, which has been fixed in monodroid/master (which I was testing on -- sorry) but exists in XA 4.8.x.
Comment 7 Grigory (Playtika) 2013-12-10 13:31:04 UTC
This is great. What can we do to get this fix ASAP ? This is quite critical for the application release. AFAIK my company has Ultimate license, can we somehow use it ?
Comment 8 Grigory (Playtika) 2015-07-13 15:53:18 UTC
Doesn't reproduce anymore on 5.0+