Mutex in C# - Synchronization from Unsynchronized Block of Code
async-await, c#, multithreading
Solution
You cannot use thread-affine synchronization primitives (e.g., a `Mutex`) with `async` in the general case. Specifically, your code after the `await` is not guaranteed to run on the same thread as the thread that acquired the mutex.
I recommend using `SemaphoreSlim` and its `WaitAsync` method. `SemaphoreSlim` is the only BCL synchronization primitive that is guaranteed to work correctly with `await`. If you need more complex synchronization primitives, Stephen Toub wrote a series of blog posts on the subject that I extended and included in my AsyncEx library.
Problem
I am writing a progam which downloads games from a website as zip archives and extracts them. Since all games are downloaded to the same location before being extracted I am using a Mutex to prevent race conditions when two downloads are going on at the same time. The issue is that I am getting a System.ApplicationException thrown saying: "Object synchronization method was called from an unsynchronized block of code." Everything I have looked at says that this is caused by freeing a Mutex from a thread that does not own it. However, my code Wait's and Releases the Mutex in the same thread. ``` await Task.Run(async () => { mutex.WaitOne(); await DownloadGameAsync(ug, nvm); await ExtractGameAsync(ug, nvm); nvm.Action = "Done"; mutex.ReleaseMutex(); }); ``` If I place a breakpoint on the line mutex.ReleaseMutex(); and step over the line it does not cause the exception to be thrown however, removing the breakpoint and running normally causes an exception with every execution. Should I be approaching this differently (Semaphores) or is my use of async/await causing the issue?