Bug 2513 - UnobservedTaskException should not be called from TaskCompletionSource.SetException
Summary: UnobservedTaskException should not be called from TaskCompletionSource.SetExc...
Alias: None
Product: iOS
Classification: Xamarin
Component: Xamarin.iOS.dll ()
Version: 7.2.0
Hardware: PC Mac OS
: --- normal
Target Milestone: Untriaged
Assignee: Jérémie Laval
Depends on:
Reported: 2011-12-15 01:27 UTC by Jahmai
Modified: 2016-02-05 16:16 UTC (History)
5 users (show)

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

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Description Jahmai 2011-12-15 01:27:23 UTC
The UnobservedTaskException event on System.Threading.Tasks.TaskScheduler is designed as a last-chance opportunity to observe an exception thrown during execution of a Task before the Finalizer thread throws it as an unhanded exception and essentially kills the app.

It appears that by using TaskCompletionSource and calling the SetException method, that the UnobservedTaskException event is fired immediately when in fact the correct behaviour is to mark the exception as unhanded internally, and the only call the UnobservedTaskException event upon Finalization.
Comment 1 Rolf Bjarne Kvinge [MSFT] 2011-12-20 03:35:12 UTC
Jérémie, this looks like something you might know about.
Comment 2 Jérémie Laval 2012-02-03 10:27:36 UTC

Could you provide a test case or relevant snippet of your program where this behavior happen? I couldn't reproduce it on simulator.

Comment 3 Jahmai 2012-02-27 07:02:36 UTC
Sorry for the late reply. I will re-test this with the latest MonoTouch and submit a snippet if it is still reproducible.
Comment 4 Jérémie Laval 2012-11-15 10:25:29 UTC
Closing since it has been a long time. Don't hesitate to reopen if that bug still exists.
Comment 5 Jahmai 2014-03-28 02:02:46 UTC
Reopening this as it is happening in iOS 7.2.0.

This function exemplifies the issue:

        private async Task AwaitOnTcsTask()
            TaskScheduler.UnobservedTaskException += (sender, e) => { Console.WriteLine("You should not see this!"); };

            TaskCompletionSource<object> tcs = new TaskCompletionSource<object>();

            tcs.SetException(new Exception());

                await tcs.Task;
            catch (Exception)
Comment 6 Rolf Bjarne Kvinge [MSFT] 2016-02-05 16:16:48 UTC
I can't reproduce this with the code from comment #5, so it looks like it's been fixed (probably when importing the reference source from Microsoft).

My test project: https://github.com/rolfbjarne/TestApp/tree/bug2513

If you can still reproduce, please provide a complete test project.