Example of using a Mutex to lock some async file system calls in WP8.

FileStorage.cs

public class FileStorage : IDisposable {
	private static readonly Dictionary<string, Mutex> Mutexes = new Dictionary<string, Mutex>(); 

	public async Task DoSomethingAsync(string accountId) {
		var mutex = GetMutex(accountId);
		try {
			mutex.WaitOne();
			
			// Do something to files
		}
		finally {
			mutex.ReleaseMutex();
		}
	}

	private static Mutex GetMutex(string accountId) {
		if (Mutexes.ContainsKey(accountId)) {
			return Mutexes[accountId];
		}

		var mutex = new Mutex(false, accountId);
		Mutexes.Add(accountId, mutex);
		return mutex;
	}

	public void Dispose() {
		foreach (var mutex in Mutexes) {
			mutex.Value.Dispose();
		}
		Mutexes.Clear();
	}
}