Async/Await–.ConfigureAwait(false)

On one of my projects, a developer asked my help when he noticed that the application just hang after invoking a specific action on a Web API controller.

In this at first innocent looking piece of code he was using a combination of async and non-async code. Here is a simplified example;

	public class SampleController
	{
	public ActionResult Index()
	{
	return View(DoSomethingAsync().Result);
	}
	private async Task<string> DoSomethingAsync()
	{
	await Task.Delay(1000);
	return "Hello async!";
	}
	}

view raw SampleController.cs hosted with ❤ by GitHub

If you ran the code above then you end up with a deadlock. This is what happens:

1. Thread "A" would be given to the request to run on and "Index" would be called
2. Thread "A" would call "DoSomethingAsync" and get a Task reference
3. Thread "A" would then request the ".Result" property of that task and would block until the task completed
4. The "Task.Delay" call would complete and the runtime would try to continue the "DoSomethingAsync" work
5. The ASP.NET synchronization context would require that work continue on Thread "A" and so the work would be placed on a queue for Thread "A".

Thread "A". is waiting until the DoSomethingAsync task is completed, but the DoSomethingAsync cannot complete because it needs Thread "A". So we end up with a deadlock situation.

The root cause is that ASP.NET applications have a special synchronization context, that returns to the same thread after an async call complete

To avoid this kind of issues it’s a good idea to set .ConfigureAwait(false). By doing this you no longer use the ASP.NET synchronization context, meaning that the work can continue on any thread available when the async work has completed.

Remark: Be aware that this requires your code to be thread-safe.

	public class SampleController
	{
	public ActionResult Index()
	{
	return View(DoSomethingAsync().Result);
	}
	private async Task<string> DoSomethingAsync()
	{
	await Task.Delay(1000).ConfigureAwait(false);
	return "Hello async!";
	}
	}

view raw SampleControllerFixed.cs hosted with ❤ by GitHub

As most of the time you don’t care on which thread the work continues, it can be a good idea to always add ".ConfigureAwait(false)" to your await calls. If you don’t want to forget this, you can use the following analyzer in your projects: https://www.nuget.org/packages/ConfigureAwaitChecker.Analyzer. This will give you a warning when a possible ConfigureAwait(false) call is missing. The analyzer is quite smart and detects when a configureawait is possible.

Remark: When you are using ASP.NET Core, you don’t have to worry about this as the synchronization context that was used for previous versions of ASP.NET is gone.