Why can’t I use the ‘await’ operator within the body of a lock statement?

by

I assume this is either difficult or impossible for the compiler team to implement for some reason.

No, it is not at all difficult or impossible to implement — the fact that you implemented it yourself is a testament to that fact. Rather, it is an incredibly bad idea and so we don’t allow it, so as to protect you from making this mistake.

call to Monitor.Exit within ExitDisposable.Dispose seems to block indefinitely (most of the time) causing deadlocks as other threads attempt to acquire the lock. I suspect the unreliability of my work around and the reason await statements are not allowed in lock statement are somehow related.

Correct, you have discovered why we made it illegal. Awaiting inside a lock is a recipe for producing deadlocks.

I’m sure you can see why: arbitrary code runs between the time the await returns control to the caller and the method resumes. That arbitrary code could be taking out locks that produce lock ordering inversions, and therefore deadlocks.

Worse, the code could resume on another thread (in advanced scenarios; normally you pick up again on the thread that did the await, but not necessarily) in which case the unlock would be unlocking a lock on a different thread than the thread that took out the lock. Is that a good idea? No.

I note that it is also a “worst practice” to do a yield return inside a lock, for the same reason. It is legal to do so, but I wish we had made it illegal. We’re not going to make the same mistake for “await”.

More Related Contents:

  1. How and when to use ‘async’ and ‘await’
  2. What is the purpose of “return await” in C#?
  3. Should I avoid ‘async void’ event handlers?
  4. Awaiting multiple Tasks with different results
  5. Using async/await for multiple tasks
  6. How do yield and await implement flow of control in .NET?
  7. Fire and forget async method in asp.net mvc
  8. TAP global exception handler
  9. Regarding usage of Task.Start() , Task.Run() and Task.Factory.StartNew()
  10. Is there anything like asynchronous BlockingCollection?
  11. Cancelling a pending task synchronously on the UI thread
  12. Does the use of the “Async” suffix in a method name depend on whether the ‘async’ modifier is used?
  13. Why use Async/await all the way down
  14. A reusable pattern to convert event into task
  15. Can the Oracle managed driver use async/await properly?
  16. ConfigureAwait pushes the continuation to a pool thread
  17. How can I prevent synchronous continuations on a Task?
  18. Async await vs GetAwaiter().GetResult() and callback
  19. How does async works in C#?
  20. Async always WaitingForActivation
  21. What is the correct way to cancel an async operation that doesn’t accept a CancellationToken?
  22. Async thread body loop, It just works, but how?
  23. Horrible performance using SqlCommand Async methods with large data
  24. The lack of non-capturing Task.Yield forces me to use Task.Run, why follow that?
  25. Does SemaphoreSlim (.NET) prevent same thread from entering block?
  26. Await and SynchronizationContext in a managed component hosted by an unmanaged app
  27. Timeout pattern on task-based asynchronous method in C#
  28. Channels with CancellationTokenSource with timeout memory leak after dispose
  29. Create a completed Task
  30. How can I run both of these methods ‘at the same time’ in .NET 4.5?

Leave a Comment