Best practice to call ConfigureAwait for all server-side code

by

Update: ASP.NET Core does not have a SynchronizationContext. If you are on ASP.NET Core, it does not matter whether you use ConfigureAwait(false) or not.

For ASP.NET “Full” or “Classic” or whatever, the rest of this answer still applies.

Original post (for non-Core ASP.NET):

This video by the ASP.NET team has the best information on using async on ASP.NET.

I had read that it is more performant since it doesn’t have to switch thread contexts back to the original thread context.

This is true with UI applications, where there is only one UI thread that you have to “sync” back to.

In ASP.NET, the situation is a bit more complex. When an async method resumes execution, it grabs a thread from the ASP.NET thread pool. If you disable the context capture using ConfigureAwait(false), then the thread just continues executing the method directly. If you do not disable the context capture, then the thread will re-enter the request context and then continue to execute the method.

So ConfigureAwait(false) does not save you a thread jump in ASP.NET; it does save you the re-entering of the request context, but this is normally very fast. ConfigureAwait(false) could be useful if you’re trying to do a small amount of parallel processing of a request, but really TPL is a better fit for most of those scenarios.

However, with ASP.NET Web Api, if your request is coming in on one thread, and you await some function and call ConfigureAwait(false) that could potentially put you on a different thread when you are returning the final result of your ApiController function.

Actually, just doing an await can do that. Once your async method hits an await, the method is blocked but the thread returns to the thread pool. When the method is ready to continue, any thread is snatched from the thread pool and used to resume the method.

The only difference ConfigureAwait makes in ASP.NET is whether that thread enters the request context when resuming the method.

I have more background information in my MSDN article on SynchronizationContext and my async intro blog post.

More Related Contents:

  1. await vs Task.Wait – Deadlock?
  2. Parallel foreach with asynchronous lambda
  3. Catch an exception thrown by an async void method
  4. Awaiting multiple Tasks with different results
  5. Running multiple async tasks and waiting for them all to complete
  6. Using async/await for multiple tasks
  7. Calling async method synchronously
  8. Parallel.ForEach and async-await [duplicate]
  9. StaTaskScheduler and STA thread message pumping
  10. TAP global exception handler
  11. How to safely call an async method in C# without await
  12. Regarding usage of Task.Start() , Task.Run() and Task.Factory.StartNew()
  13. Why is HttpContext.Current null after await?
  14. Task sequencing and re-entracy
  15. Difference between await and ContinueWith
  16. Is there anything like asynchronous BlockingCollection?
  17. Cancelling a pending task synchronously on the UI thread
  18. ConfigureAwait pushes the continuation to a pool thread
  19. How can I prevent synchronous continuations on a Task?
  20. multiple awaits vs Task.WaitAll – equivalent?
  21. Use Task.Run() in synchronous method to avoid deadlock waiting on async method?
  22. What is the correct way to cancel an async operation that doesn’t accept a CancellationToken?
  23. How to throttle multiple asynchronous tasks?
  24. The lack of non-capturing Task.Yield forces me to use Task.Run, why follow that?
  25. can not await async lambda
  26. Task.WhenAny – What happens with remaining running tasks?
  27. Transform IEnumerable asynchronously by awaiting each task
  28. How to implement Task.WhenAny() with a predicate
  29. Create a completed Task
  30. How to construct a Task without starting it?

Leave a Comment