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.

Answer 1

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.