ReaderWriterLockSlim and async\await

Question

I have some problems with ReaderWriterLockSlim. I cannot understand how it's magic working.

My code:

 private async Task LoadIndex()
    {
        if (!File.Exists(FileName + ".index.txt"))
        {
            return;
        }
        _indexLock.EnterWriteLock();// <1>
        _index.Clear();
        using (TextReader index = File.OpenText(FileName + ".index.txt"))
        {
            string s;
            while (null != (s = await index.ReadLineAsync()))
            {
                var ss = s.Split(':');
                _index.Add(ss[0], Convert.ToInt64(ss[1]));
            }
        }
        _indexLock.ExitWriteLock();<2>
    }

When I enter write lock at <1>, in debugger I can see that _indexLock.IsWriteLockHeld is true, but when execution steps to <2> I see _indexLock.IsWriteLockHeld is false and _indexLock.ExitWriteLock throws an exception SynchronizationLockException with message "The write lock is being released without being held". What I doing wrong?

Answer 1

ReaderWriterLockSlim is a thread-affine lock type, so it usually cannot be used with async and await.

You should either use SemaphoreSlim with WaitAsync, or (if you really need a reader/writer lock), use my AsyncReaderWriterLock from AsyncEx or Stephen Toub's AsyncReaderWriterLock.