HttpClient.GetAsync(…) never returns when using await/async

You are misusing the API.

Here’s the situation: in ASP.NET, only one thread can handle a request at a time. You can do some parallel processing if necessary (borrowing additional threads from the thread pool), but only one thread would have the request context (the additional threads do not have the request context).

This is managed by the ASP.NET SynchronizationContext.

By default, when you await a Task, the method resumes on a captured SynchronizationContext (or a captured TaskScheduler, if there is no SynchronizationContext). Normally, this is just what you want: an asynchronous controller action will await something, and when it resumes, it resumes with the request context.

So, here’s why test5 fails:

• Test5Controller.Get executes AsyncAwait_GetSomeDataAsync (within the ASP.NET request context).
• AsyncAwait_GetSomeDataAsync executes HttpClient.GetAsync (within the ASP.NET request context).
• The HTTP request is sent out, and HttpClient.GetAsync returns an uncompleted Task.
• AsyncAwait_GetSomeDataAsync awaits the Task; since it is not complete, AsyncAwait_GetSomeDataAsync returns an uncompleted Task.
• Test5Controller.Get blocks the current thread until that Task completes.
• The HTTP response comes in, and the Task returned by HttpClient.GetAsync is completed.
• AsyncAwait_GetSomeDataAsync attempts to resume within the ASP.NET request context. However, there is already a thread in that context: the thread blocked in Test5Controller.Get.
• Deadlock.

Here’s why the other ones work:

• (test1, test2, and test3): Continuations_GetSomeDataAsync schedules the continuation to the thread pool, outside the ASP.NET request context. This allows the Task returned by Continuations_GetSomeDataAsync to complete without having to re-enter the request context.
• (test4 and test6): Since the Task is awaited, the ASP.NET request thread is not blocked. This allows AsyncAwait_GetSomeDataAsync to use the ASP.NET request context when it is ready to continue.

And here’s the best practices:

1. In your “library” async methods, use ConfigureAwait(false) whenever possible. In your case, this would change AsyncAwait_GetSomeDataAsync to be var result = await httpClient.GetAsync("http://stackoverflow.com", HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false);
2. Don’t block on Tasks; it’s async all the way down. In other words, use await instead of GetResult (Task.Result and Task.Wait should also be replaced with await).

That way, you get both benefits: the continuation (the remainder of the AsyncAwait_GetSomeDataAsync method) is run on a basic thread pool thread that doesn’t have to enter the ASP.NET request context; and the controller itself is async (which doesn’t block a request thread).

More information:

• My async/await intro post, which includes a brief description of how Task awaiters use SynchronizationContext.
• The Async/Await FAQ, which goes into more detail on the contexts. Also see Await, and UI, and deadlocks! Oh, my! which does apply here even though you’re in ASP.NET rather than a UI, because the ASP.NET SynchronizationContext restricts the request context to just one thread at a time.
• This MSDN forum post.
• Stephen Toub demos this deadlock (using a UI), and so does Lucian Wischik.

Update 2012-07-13: Incorporated this answer into a blog post.