Task and Task.WaitAll with timeout exception handling

Question

When waiting for Tasks using Task.WaitAll and specifying a timeout if the WaitAll times out do I also have to separately observe any unfinished tasks (e.g. by registering a continuation)?

This thread leads me to think the answer is yes, but I haven't found anything else that confirms this.

var random = new Random();
var tasks = Enumerable.Range(0, 10).Select((i) => Task.Factory.StartNew(() => {
        // Sleep 5-15 seconds
        Thread.Sleep(random.Next(5000, 15000));
        throw new Exception("Some exception.");
    }
)).ToArray();

try {
    // Wait for 10 seconds
    Task.WaitAll(tasks, 10000);
} catch (AggregateException) {
    // Swallow
}

// Check to see how many tasks were unfinished
tasks.Where(t => !t.IsCompleted).Count().Dump("Unfinished count: ");

// Is this neccessary to avoid exceptions being thrown on the finalizer?
foreach (Task task in tasks) {
    task.ContinueWith(t => t.Exception, TaskContinuationOptions.OnlyOnFaulted);
}

How many exceptions are present in the AggregateException you catch after WaitAll? — Dan Bryant, Aug 18 '11 at 18:37
The complete simple lib source here: http://stackoverflow.com/questions/11831844/unobservedtaskexception-being-throw-but-it-is-handled-by-a-taskscheduler-unobser — , Jan 21 '13 at 15:30

score 2 · Answer 1 · edited Apr 09 '15 at 18:28

2

To avoid crashing the finalizer, you have to observe the exceptions thrown by the Task body. To observe a Task exception you have to do one of the following:

Access the exception property
Call Task.Wait/WaitAll/WaitAny
Register a continuation that only run if the task faulted

What you did is definitely necessary to avoid crashing the finalizer.

edited Apr 09 '15 at 18:28

Michael

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answered Aug 23 '11 at 00:55

Emad Omara

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score 1 · Accepted Answer · answered Aug 18 '11 at 16:52

Testing with LINQPad v4.31 indicates the answer is yes, since if you comment out the foreach you get an unhandled exception dialog for each unfinished task.

I believe I understand why this is at this point but if anybody wants to have a go at a more technical explanation of this behavior by all means go for it.

Task and Task.WaitAll with timeout exception handling

2 Answers2

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