Creating the AsyncSemaphore

Let's add a new folder called Threading to the FileStorage.Portable project. Inside this folder, we are going to add a new file called AsyncSemaphore.cs and implement the first part as follows:

public class AsyncSemaphore
    {
        private readonly static Task s_completed = Task.FromResult(true);
        private readonly Queue<TaskCompletionSource<bool>> m_waiters = new Queue<TaskCompletionSource<bool>>();
        private int m_currentCount;
        public AsyncSemaphore(int initialCount)
        {
           if (initialCount < 0) throw new ArgumentOutOfRangeException("initialCount");
           m_currentCount = initialCount; 
        }
        public Task WaitAsync()
        {
           lock (m_waiters)
            {
               if (m_currentCount > 0)
                 {
                    --m_currentCount;
                    return s_completed;
                 }
               else
                {
  ...