SynchronizationLockException on Monitor.Exit when using await

Question

I am creating a piece of code that gets a webpage from a legacy system we have. In order to avoid excessive querying, I am caching the obtained URL. I am using Monitor

Answer 1

You may use interlocked class to simulate the lock statement, here is the code:

    private async Task OpenReport(String report)
    {
        var file = _directory.GetFiles(report + ".html");
        if (file != null && file.Any())
            return file[0].OpenRead();
        else
        {
            object locker = _locker;
            try
            {
                while (locker == null || Interlocked.CompareExchange(ref _locker, null, locker) != locker)
                {
                    await Task.Delay(1);
                    locker = _locker;
                }
                FileInfo newFile = new FileInfo(Path.Combine(_directory.FullName, report + ".html"));
                if (!newFile.Exists) // Double check
                {
                    using (var target = newFile.OpenWrite())
                    {
                        WebRequest request = WebRequest.Create(BuildUrl(report));
                        var response = await request.GetResponseAsync();
                        using (var source = response.GetResponseStream())
                            source.CopyTo(target);
                    }
                }
                return newFile.OpenRead();
            }
            finally
            {
                _locker = locker;
            }
        }
    }