C# Lock and Async Method

Question

I am not clear (and can't find documentation clear enough): when using the lock keyword in an async method: will the thread be blocked if the object is already blocked or will it return a task in suspended state (not blocking the thread, and returning when the lock is released)?

In the code below, will the line block the thread? If it blocks the thread (which is what I think), is there an standard not blocking solution? I am considering using AsyncLock, but first I wanted to try for something standard.

thanks.

private object myLock = new object();   private async Task MyMethod1(){   lock (myLock ) {// <---- will this line cause a return of the current method as an Await method call would do if myLock was already locked.       ....    } }  // other methods that lock on myLock 

Answer 1

In the code below, will the line block the thread?

Technically, yes, but it won't work as you expect.

There are two reasons why thread-affine locks don't play well with async. One is that (in the general case), an async method may not resume on the same thread, so it would try to release a lock it doesn't own while the other thread holds the lock forever. The other reason is that during an await while holding a lock, arbitrary code may execute while the lock is held.

For this reason, the compiler goes out of its way to disallow await expressions within lock blocks. You can still shoot yourself in the foot by using Monitor or other primitives directly, though.

If it blocks the thread (which is what I think), is there an standard not blocking solution?

Yes; the SemaphoreSlim type supports WaitAsync.

Answer 2

This has been disallowed to stop deadlocks (i.e. developers hurting themselves). The best solution I've found is to use semaphores - See this post for details.

Relevant code extract:

static SemaphoreSlim semaphoreSlim = new SemaphoreSlim(1, 1);  ...  await semaphoreSlim.WaitAsync(); try {     await Task.Delay(1000); } finally {     semaphoreSlim.Release(); }