Can a Task have multiple awaiters?

Question

I am toying around with an async service for a Windows 8 project and there are some async calls of this service, which should only be called once at a time.

Answer 1

This code looks very "racy" if multiple threads might be involved.

One example (I'm sure there are more). Assume that _callThisOnlyOnce is currently null:

Thread 1                                                          Thread 2

public Task CallThisOnlyOnce()
{
  if(_callThisOnlyOnce != null && _callThisOnlyOnce.IsCompleted)
     _callThisOnlyOnce = null;

  if(_callThisOnlyOnce == null)
                                                                   public Task CallThisOnlyOnce()
                                                                   {
                                                                     if(_callThisOnlyOnce != null && _callThisOnlyOnce.IsCompleted)
                                                                        _callThisOnlyOnce = null;

                                                                     if(_callThisOnlyOnce == null)
                                                                        _callThisOnlyOnce = CallThisOnlyOnceAsync();

                                                                     return _callThisOnlyOnce;
                                                                   }
     _callThisOnlyOnce = CallThisOnlyOnceAsync();

  return _callThisOnlyOnce;
}

You now have 2 calls running simultaneously.

As to multiple awaiters, yes you can do this. I'm sure I've seen example code from MS somewhere showing an optimization where e.g. the result of Task.FromResult(0) is stored away in a static member and returned any time the function wants to return zero.

I have, however, been unsuccessful in locating this code sample.