When can an AggregateException contain more than 1 inner exception?

Question

I have a question regarding the .NET Task Parallel Library's error handling. In which cases will an AggregateException hold more than 1 inner exception? I know this can happen for ex. when calling Task.WaitAll(anArrayOfTasks), and 2 or more tasks are throwing an exception, but are there any other cases (i.e., can it be possible that, if only waiting for 1 task to finish, you get more than 1 inner exceptions)?

Answer 1

A task can raise an aggregate exception which inherently can contain multiple inner exceptions. This means that you should always consider an aggregate exception with multiple inner exceptions when working with tasks. Even if you're not using Task.WaitAll, the task you're waiting on might internally wait for multiple subtasks. Alternatively, the task your waiting on might return multiple exceptions. You simply can't know as a caller.

Answer 2

This can happen if you have a “parent” task and one or more “child” tasks that are attached to the parent. What that means is that the parent task will finish only when all of its child tasks finish and the exceptions from child tasks are also propagated to the parent task.

Take, for example, the following code:

var task = Task.Factory.StartNew(
    () =>
    {
        Task.Factory.StartNew(
            () => { throw new Exception("inner"); },
            TaskCreationOptions.AttachedToParent);

        throw new Exception("outer");
    });

If you Wait() on that task, it thows an AggregateException, that looks like this:

• AggregateException
  • Exception: outer
  • AggregateException
    • Exception: inner

If you don't like that it can contain AggregateExceptions inside AggregateExceptions, you can use the Flatten() method. There's also another method that can be used for processing of AggregateExceptions: Handle().