Is asynchronous file IO like FileStream.BeginWrite thread safe? If not, then it need to wrap with "SyncLock", that's mean it is still blocking thread.
FileStream.BeginWrite already starts a new thread to access the files. You don't need to start the FileStream.BeginWrite on a separate thread (because that will be thread in a thread). On the other hand, multiple FileStream.BeginWrite functions should not be accessing the same file at the same time.