Question

How do you start multiple HttpClient.GetAsync() requests at once, and handle them each as soon as their respective responses come back? First what I tried is:

var response1 = await client.GetAsync("http://example.com/");
var response2 = await client.GetAsync("http://stackoverflow.com/");
HandleExample(response1);
HandleStackoverflow(response2);

But of course it’s still sequential. So then I tried starting them both at once:

var task1 = client.GetAsync("http://example.com/");
var task2 = client.GetAsync("http://stackoverflow.com/");
HandleExample(await task1);
HandleStackoverflow(await task2);

Now the tasks are started at the same time, which is good, but of course the code still has to wait for one after the other.

What I want is to be able to handle the “example.com” response as soon as it comes in, and the “stackoverflow.com” response as soon as it comes in.

I could put the two tasks in an array an use Task.WaitAny() in a loop, checking which one finished and call the appropriate handler, but then … how is that better than just regular old callbacks? Or is this not really an intended use case for async/await? If not, how would I use HttpClient.GetAsync() with callbacks?

To clarify — the behaviour I’m after is something like this pseudo-code:

client.GetAsyncWithCallback("http://example.com/", HandleExample);
client.GetAsyncWithCallback("http://stackoverflow.com/", HandleStackoverflow);

Answer 1

You can use ContinueWith and WhenAll to await one new Task, task1 and task2 will be executed in parallel

var task1 = client.GetAsync("http://example.com/")
                  .ContinueWith(t => HandleExample(t.Result));

var task2 = client.GetAsync("http://stackoverflow.com/")
                  .ContinueWith(t => HandleStackoverflow(t.Result));

var results = await Task.WhenAll(new[] { task1, task2 });