5

I have a method which is called frequently from multiple threads. It involves writing to disk using await FileIO.WriteTextAsync. This works fine when it is called from a single thread, but once I start doing this in multiple threads, I get this error:

The process cannot access the file because it is being used by another process.

I know what the error means, but I'm not sure how to work around it. Normally, I would create a lock(object) statement, to ensure that the file is being accessed by only one thread at a time. However, this is an asynchronous method and as such I can't use the await operator in the body of the lock(object) statement.

Please advise on how to handle this scenario.

c0D3l0g1c
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1 Answers1

6

You can use SemaphoreSlim to act as an async-compatible lock:

SemaphoreSlim _mutex = new SemaphoreSlim(1);

async Task MyMethodAsync()
{
  await _mutex.WaitAsync();
  try
  {
    ...
  }
  finally
  {
    _mutex.Release();
  }
}

Personally, I don't like the finally, so I usually write my own IDisposable to release the mutex when disposed, and my code can look like this:

async Task MyMethodAsync()
{
  // LockAsync is an extension method returning my custom IDisposable
  using (await _mutex.LockAsync()) 
  {
    ...
  }
}
Stephen Cleary
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  • Well Stephen it's no wonder why you have such a massive reputation. The solution and your personal touch to it was fantastic. My process is running perfectly now. I've also used your suggestion to beef up my logging class which was suffering from a similar problem. Thanks for that! – c0D3l0g1c Jun 08 '13 at 14:26