Task.WaitAll and Exceptions

Question

I have a problem with exception handling and parallel tasks.

The code shown below starts 2 tasks and waits for them to finish. My problem is, that in case a task throws an exception, the catch handler is never reached.

        List<Task> tasks = new List<Task>();         try         {                             tasks.Add(Task.Factory.StartNew(TaskMethod1));             tasks.Add(Task.Factory.StartNew(TaskMethod2));              var arr = tasks.ToArray();                             Task.WaitAll(arr);         }         catch (AggregateException e)         {             // do something         } 

However when I use the following code to wait for the tasks with a timeout, the exception is caught.

 while(!Task.WaitAll(arr,100)); 

I seem to be missing something, as the documentation for WaitAll describes my first attempt to be the correct one. Please help me in understanding why it is not working.

Answer 1

Can't reproduce this - it works fine for me:

using System; using System.Threading; using System.Threading.Tasks;  class Test {     static void Main()     {         Task t1 = Task.Factory.StartNew(() => Thread.Sleep(1000));         Task t2 = Task.Factory.StartNew(() => {             Thread.Sleep(500);             throw new Exception("Oops");         });          try         {             Task.WaitAll(t1, t2);             Console.WriteLine("All done");         }         catch (AggregateException)         {             Console.WriteLine("Something went wrong");         }     } } 

That prints "Something went wrong" just as I'd expect.

Is it possible that one of your tasks isn't finished? WaitAll really does wait for all the tasks to complete, even if some have already failed.

Answer 2

Here's how I solved the problem, as alluded to in the comments on my answer/question (above):

The caller catches any exceptions raised by the tasks being coordinated by the barrier, and signals the other tasks with a forced cancellation:

CancellationTokenSource cancelSignal = new CancellationTokenSource(); try {     // do work     List<Task> workerTasks = new List<Task>();     foreach (Worker w in someArray)     {         workerTasks.Add(w.DoAsyncWork(cancelSignal.Token);     }     while (!Task.WaitAll(workerTasks.ToArray(), 100, cancelSignal.Token)) ;   }  catch (Exception)  {      cancelSignal.Cancel();      throw;  }