Await Task.WhenAll() inside task not awaiting

Question

My problem is that when a Task has a Task.WhenAll() call (running other Tasks) the line of WhenAll() makes the consuming code continue execution, unlike what I would expect. So the following code outputs "finished" immediately when the Task.WhenAll() is hit, not after all the tasks in its argument are finished.

    // Just a simple async method
    public Task DoWorkAsync()
    {
        return Task.Factory.StartNew(
            () =>
            {
                // Working
            });
    }

    // This one used the previous one with Task.WhenAll()
    public Task DoLoadsOfWorkAsync()
    {
        return Task.Factory.StartNew(
            async () =>
            {
                // Working
                // This line makes the task return immediately
                await Task.WhenAll(DoWorkAsync(), DoWorkAsync());
                // Working
            });
    }

    // Consuming code
    await DoLoadsOfWorkAsync();
    Console.WriteLine("finished");

I'd expect the WriteLine() to be called when the last line of DoLoadsOfWorkAsync() is executed.

What am I doing wrong? Thanks in advance.

Solution

Task.WhenAll returns a new Task immediately, it does not block. The returned task will complete when all tasks passed to WhenAll have completed.

It is an asynchronous equivalent to Task.WaitAll, and this is the method to use if you want to block.

However you have another problem. Using Task.Factory.StartNew and passing an async delegate seems to lead to a type of Task<Task> where the outer task completes when the inner task starts to execute (rather than when it has completed).

Using the newer Task.Run avoids this.

Comments

[Piedone]

You nailed it: the problem was the Task<Task>, I have overlooked it. BTW I don't want the thread to block, just to continue execution after the previous tasks are finished. That's why I used Task.WhenAll() instead of wait together with the aync-await construct. Thank you very much for your help!

[ckozl]

if you want to read about Task.Run and all its goodness (and why and how it does what it does) go here: blogs.msdn.com/b/pfxteam/archive/2011/10/24/10229468.aspx