Async and Await with HttpWebRequest.GetResponseAsync

Question

I am trying to use Async and Await when making a web request and am finding that it never gets past the await line. I am doing this from a Metro app, but I also verified the problem in a winforms app.

public async Task<string> DoSomething()
{
    string url = "http://imgur.com/gallery/VcBfl.json";
    HttpWebRequest request = HttpWebRequest.CreateHttp(url);

    var ws = await request.GetResponseAsync();

    return ws.ResponseUri.ToString(); ;
}

If I don't use await and instead perform a synchronous wait, it works, but I need this to run asynchronously.

What am I missing in this code that is causing the await to never return?

weston · Accepted Answer · 2015-07-10 08:19:40Z

39

I suspect that further up your call stack, you're either calling Wait or Result on the returned Task. This will cause a deadlock, as I describe on my blog.

Follow these best practices to avoid the deadlock:

Don't block on async code; use async all the way down.
In your "library" methods, use ConfigureAwait(false).

edited Jul 10, 2015 at 8:19

weston

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answered Oct 3, 2012 at 4:31

Stephen Cleary

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5 Comments

John Koerner Over a year ago

Thank you. That was exactly it. Thanks for the link, it is very helpful.

Brad Over a year ago

@Stephen, thanks for the blog posts! Clarifying question: if I use .ConfigureAwait(false) in my lib (basically a WebClient), can I then use .Result when calling it? It seems to work, but I don't know what will happen when I set it loose in the wild.

Stephen Cleary Over a year ago

@Brad: Yes, as long as the async method you're calling does the same thing. However, it's not a best practice to use Result; there are always better options.

Brad Over a year ago

@Stephen, the async method I'm calling returns a Task and does NOT call result. The code in question is a custom wrapper for calling an internal API and the only part I want to make async in the actual GET/POST methods (GetRequestStreamAsync, GetResponseAsync, etc). How do I keep this async without making every method between this and the controller action async and returning a Task? I'd like for the lib to take care of the async nature and the consumer to remain agnostic.

Stephen Cleary Over a year ago

@Brad: That's not possible, due to the nature of asynchronous code. It's best to use async all the way.

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