Task.WhenAll().ContinueWith() throws unexpected Task Canceled exception

Question

I am using Task.WhenAll to schedule tasks concurrently (more or less of course) and want to rethrow all exceptions rather than the first one what Task.WhenAll would do. So I want to perfor a continuation whenever any child task faulted or has been canceled. That's what NotOnRanToCompletion means, right?

Well this actually works but in case that all Tasks actually have completed, I get a Task Cancelled exception on the Continuation?

var semaphore = new SemaphoreSlim(10);
var tasks = portList.Select(async port =>
{
    try
    {
        await semaphore.WaitAsync();
        await this.AddDeviceAsync(bioprocessDevicesDto, template, port);
    }
    finally
    {
        semaphore.Release();
    }
});
await Task.WhenAll(tasks).ContinueWith(t =>
{
    // this will make all inner exceptions available
    // never null
    throw t.Exception;
}, TaskContinuationOptions.NotOnRanToCompletion).ConfigureAwait(false);

From the documentation: "If the criteria specified through the `continuationOptions` parameter are not met, the continuation task will be canceled instead of scheduled." — Damien_The_Unbeliever, Apr 22 '20 at 09:30
oh boy really? I did not find that. I must have been asleep. Well that pretty much explains it. I actually did not expect this kind of behaviour but well what can I say — yBother, Apr 22 '20 at 09:31
In this case I will use TaskContinuationOptions.None and rethow Exception if !=null. Is checking for Task.Status!=RanToCompletion necessary here? — yBother, Apr 22 '20 at 09:35
another option is to actually write `catch` block and hendle exception there, so your task will run to its end — Sina Hoseinkhani, Apr 22 '20 at 09:54

score 2 · Answer 1 · answered Apr 24 '20 at 11:29

2

That behavior is by-design and documented.

Note that your code is not awaiting the task returned from WhenAll; it's awaiting the continuation from ContinueWith.

ContinueWith is a low-level method with dangerous default behavior. Use await instead:

var tasks = portList.Select(...);
var allTask = Task.WhenAll(tasks);
try { await allTask; }
catch { throw allTask.Exception; }

answered Apr 24 '20 at 11:29

Stephen Cleary

437,863
77
675
810

hmm that's interesting. all my tests worked as expected with the .ContinueWith() approach. on the other hand my tests revealed that (your) classic try{}catch{} approach just behaves the same like withouht try{}catch{} : only the very first exception will be thrown! how do I get all exceptions then? – yBother Apr 24 '20 at 19:00
1

It works fine for me. Without the `try`/`catch`, the first exception is raised from this code; with the `try`/`catch`, an `AggregateException` is raised from this code. – Stephen Cleary Apr 24 '20 at 19:45
1

Yes it does work fine. I made a mistake beforel. I will move this try catch logic to an Task Extension Method 'PreserveAllExceptions' that uses allTasks as parameter so I can use it like this: await Task.WhenAll(tasks).PreserveAllExceptions().ConfigureAwait(false); – yBother Apr 27 '20 at 10:52
1

Hmmm... I really like the idea of that extension method! – Stephen Cleary Apr 27 '20 at 13:28
@yBother you might find this interesting: [I want await to throw AggregateException, not just the first Exception](https://stackoverflow.com/questions/18314961/i-want-await-to-throw-aggregateexception-not-just-the-first-exception). – Theodor Zoulias Jun 13 '22 at 09:53

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

1

When you look at the TaskContinuationOptions for NotOnRanToCompletion, it states:

This option is not valid for multi-task continuations.

I would try changing NotOnRanToCompletion to another option that handles multi-task continuations.

edited Jun 20 '20 at 09:12

Community

1
1

answered Apr 22 '20 at 10:16

Brett Wertz

412
4
19

Task.WhenAll().ContinueWith() throws unexpected Task Canceled exception

2 Answers2