Chaining two functions () -> Task and A->Task

Answer 1

Does your getB have to be a method which returns Task rather than B?

The problem is that ContinueWith is:

public Task ContinueWith(
    Func, TNewResult> continuationFunction,
    CancellationToken cancellationToken
)

So in your case, because getB returns Task, you're passing in a Func, Task>, so TNewResult is Task.

If you can change getB to just return a B given an A, that would work... or you could use:

return ta.ContinueWith(a => getB(a.Result).Result);

Then the lambda expression will be of type, Func, B> so ContinueWith will return a Task.

EDIT: In C# 5 you could easily write:

public async Task CombinedAsync()
{
    A a = await getA();
    B b = await getB(a);
    return b;
}

... so it's "just" a matter of working out what that ends up as. I suspect it's something like this, but with error handling:

public Task CombinedAsync()
{
    TaskCompletionSource source = new TaskCompletionSource();
    getA().ContinueWith(taskA => {
        A a = taskA.Result;
        Task taskB = getB(a);
        taskB.ContinueWith(t => source.SetResult(t.Result));
    });
    return source.Task;
}

Does that make sense?