Waiting on a Task with a OnlyOnFaulted Continuation causes an AggregateException

Question

I have some simple code as a repro:

var taskTest = Task.Factory.StartNew(() =>
{
    System.Threading.Thread.Sleep(5000);

}).ContinueWith((Task t) =>
{
    Console.WriteLine("ERR");
}, TaskContinuationOptions.OnlyOnFaulted);

try
{
    Task.WaitAll(taskTest);
}
catch (AggregateException ex)
{
    foreach (var e in ex.InnerExceptions)
        Console.WriteLine(e.Message + Environment.NewLine + e.StackTrace);
}

However, I'm getting an unexpected TaskCanceledException being thrown in the try catch block (it's in the AggregateException InnerExceptions object). "A task was canceled".

Why am I getting this exception? The Continuation for the task never fires, there was no exception generated by it, yet I still get the aggregate exception when waiting....

I'm hoping someone can explain how this makes sense to me :)

Answer 1

You're not waiting on a task with an OnlyOnFaulted continuation - you're waiting on that continuation (returned by ContinueWith). The continuation is never going to fire because the original task returned normally, so it's acting as if it were cancelled.

Makes sense to me.

I suspect you want to create the task, add the continuation, but then wait on the original task:

var taskTest = Task.Factory.StartNew(() =>
{
    System.Threading.Thread.Sleep(5000);

});
taskTest.ContinueWith((Task t) =>
{
    Console.WriteLine("ERR");
}, TaskContinuationOptions.OnlyOnFaulted);