Why is only one from many exceptions from child tasks always propagated?

Question

I am struggling to better grasp the rationale of exception and error handling in TPL (and with some more luck in .NET 4.5 async/await tasks)

The slightly modified

Answer 1

You should not mix parent/child tasks with async. They were not designed to go together.

svick already answered this question as part of his (correct) answer to your other question. Here's how you can think of it:

• Each inner StartNew gets one exception, which is wrapped into an AggregateException and placed on the returned Task.
• The outer StartNew gets both AggregateExceptions from its child tasks, which it wraps into another AggregateException on its returned Task.
• When you await a Task, the first inner exception is raised. Any others are ignored.

You can observe this behavior by saving the Tasks and inspecting them after the exception is raised by await:

async static Task Test()
{
    Task containingTask, nullRefTask, argTask;
    try
    {
        containingTask = Task.Factory.StartNew(() =>
        {
            nullRefTask = Task.Factory.StartNew(() =>
            {
                throw new NullReferenceException();
            }, TaskCreationOptions.AttachedToParent);
            argTask = Task.Factory.StartNew(() =>
            {
                throw new ArgumentException();
            }, TaskCreationOptions.AttachedToParent);
        });
        await containingTask;
    }
    catch (AggregateException ex)
    {
        Console.WriteLine("** {0} **", ex.GetType().Name);
    }
}

If you put a breakpoint on WriteLine, you can see that the exceptions from both child tasks are being placed on the parent task. The await operator only propagates one of them, so that's why you only catch one.