How to create a wrapper for an async-await call?

Question

From what I can tell, there is no built-in (or framework extension) support for ConnectAsync/AcceptAsync/SendAsync/ReceiveAsync, etc.. How would I write my own wrapper that would be supported by the async-await mechanism. For example, my current code which handles a ReceiveAsync both inline and on the callback (which is specified in the SocketAsyncEventArgs):

private void PostReceive(SocketAsyncEventArgs e)
{       
    e.SetBuffer(ReceiveBuffer.DataBuffer, ReceiveBuffer.Count, ReceiveBuffer.Remaining);            
    e.Completed += Receive_Completed;

            // if ReceiveAsync returns false, then completion happened inline
    if (m_RemoteSocket.ReceiveAsync(e) == false)
    {
        Receive_Completed(this, e);
    }                          
}

.

private void Receive_Completed(object sender, SocketAsyncEventArgs e)
{   
    e.Completed -= Receive_Completed;       

    if (e.BytesTransferred == 0 || e.SocketError != SocketError.Success)
    {
        if (e.BytesTransferred > 0)
        {                   
            OnDataReceived(e);
        }

        Disconnect(e);                
        return;
    }

    OnDataReceived(e);

    //
    // we do not push the SocketAsyncEventArgs back onto the pool, instead
    // we reuse it in the next receive call
    //
    PostReceive(e);
}

Answer 1

The trick is to use TaskCompletionSource to handle this scenario.

I blogged about this. For details, see Preparing Existing code For Await.

Answer 2

You can also write a custom awaitable, which I like better in this situation. This is a technique by Stephen Toub from Microsoft. You can read more about this technique here. http://blogs.msdn.com/b/pfxteam/archive/2011/12/15/10248293.aspx

Here is the custom awaitable:

public sealed class SocketAwaitable : INotifyCompletion
{
    private readonly static Action SENTINEL = () => { };
    internal bool m_wasCompleted;
    internal Action m_continuation;
    internal SocketAsyncEventArgs m_eventArgs;
    public SocketAwaitable(SocketAsyncEventArgs eventArgs)
    {
        if (eventArgs == null) throw new ArgumentNullException("eventArgs");
        m_eventArgs = eventArgs;
        eventArgs.Completed += delegate
        {
            var prev = m_continuation ?? Interlocked.CompareExchange(
                ref m_continuation, SENTINEL, null);
            if (prev != null) prev();
        };
    }
    internal void Reset()
    {
        m_wasCompleted = false;
        m_continuation = null;
    }
    public SocketAwaitable GetAwaiter() { return this; }
    public bool IsCompleted { get { return m_wasCompleted; } }
    public void OnCompleted(Action continuation)
    {
        if (m_continuation == SENTINEL ||
            Interlocked.CompareExchange(
                ref m_continuation, continuation, null) == SENTINEL)
        {
            Task.Run(continuation);
        }
    }
    public void GetResult()
    {
        if (m_eventArgs.SocketError != SocketError.Success)
            throw new SocketException((int)m_eventArgs.SocketError);
    }
}

Some extension methods to add to the socket class and make it convenient:

public static class SocketExtensions
{
    public static SocketAwaitable ReceiveAsync(this Socket socket,
        SocketAwaitable awaitable)
    {
        awaitable.Reset();
        if (!socket.ReceiveAsync(awaitable.m_eventArgs))
            awaitable.m_wasCompleted = true;
        return awaitable;
    }
    public static SocketAwaitable SendAsync(this Socket socket,
        SocketAwaitable awaitable)
    {
        awaitable.Reset();
        if (!socket.SendAsync(awaitable.m_eventArgs))
            awaitable.m_wasCompleted = true;
        return awaitable;
    }
    // ... 
}

In use:

    static async Task ReadAsync(Socket s)
    {
        // Reusable SocketAsyncEventArgs and awaitable wrapper 
        var args = new SocketAsyncEventArgs();
        args.SetBuffer(new byte[0x1000], 0, 0x1000);
        var awaitable = new SocketAwaitable(args);

        // Do processing, continually receiving from the socket 
        while (true)
        {
            await s.ReceiveAsync(awaitable);
            int bytesRead = args.BytesTransferred;
            if (bytesRead <= 0) break;

            Console.WriteLine(bytesRead);
        }
    }