C# AggregateException, in what scenario would I have aggregated exceptions?

Question

I am probably missing a very obvious fact, but I have a hard time understanding the need for AggregatedExceptions.

I know since async/await we don't have to bother with AggregatedExceptions anymore (or at least are confronted with them less frequently). I can relate to that, because I simply start a task and at some time I choose to synchronize the 'calling thread' with the 'task which runs in parallel'

var sometTask = DoSomethingInParallelAsync().ConfigureAwait(false);
...
...
await someTask;

In this scenario, an exception that occurred in DoSomethingInParallelAsync() will be presented to me when I await that call.

Why is that so different without using await, but Wait on the Task explicitly?

var someTask = DoSomethingInParallelAsync();
...
...
someTask.Wait();

An exception thrown in this example, will always wrap the thrown Exception in an AggregateException. I still don't understand why.

What can't I do when using async/await which takes away the need for AggregateExceptions?

Hi @Sean, thanks for the link. Still puzzled though. In the example presented below I understand that multiple exceptions can occur on different threads / tasks in parallel. But in my question, I simply start 1 task, and wait for it. That wouldn't produce multiple exceptions on the Task.Wait call. So why is it still wrapped in the AggregateException? — bas
– bas, Commented Oct 30, 2019 at 19:32
Not sure if I make myself clear. I would understand that Task.WhenAll would throw an AggregateException, I don't understand why Task.Wait would throw an AggregateException — bas
– bas, Commented Oct 30, 2019 at 19:34

Fahim Akhtar · Accepted Answer · 2019-10-30 13:54:32Z

0

Causing and handling the error

Task task1 = Task.Factory.StartNew(() => { throw new ArgumentException(); } );
Task task2 = Task.Factory.StartNew(() => { throw new UnauthorizedAccessException(); } );

try
{
    Task.WaitAll(task1, task2);
}
catch (AggregateException ae)
{
}

and also see that links

here is an example : link2

answered Oct 30, 2019 at 13:54

Fahim Akhtar

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1 Comment

bas Over a year ago

OK, that does explain why the AggregateException exists. But that does not really explain to me why the Task.Wait call would wrap the exception in a AggregateException. In what scenario would you be able to produce more than one exception on a single task? It's just one task, with one result, right? Failed with an exception or completed successfully. PS: I also don't understand who downvoted your answer either ...

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