Using .Net 4.5 Async Feature for Socket Programming

I've previously used BeginAccept() and BeginRead(), but with Visual Studio 2012 I want to make use of the new asynchronous (async, await) features in my socket server program.

How can I complete the AcceptAsync and ReceiveAsync functions?

using System.Net;
using System.Net.Sockets;

namespace OfficialServer.Core.Server
    public abstract class CoreServer
        private const int ListenLength = 500;
        private const int ReceiveTimeOut = 30000;
        private const int SendTimeOut = 30000;
        private readonly Socket _socket;

        protected CoreServer(int port, string ip = "")
            _socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            _socket.Bind(new IPEndPoint(IPAddress.Parse(ip), port));
            _socket.ReceiveTimeout = ReceiveTimeOut;
            _socket.SendTimeout = SendTimeOut;
            _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, true);
            _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.DontLinger, true);

        public void Start()


...because you're so determined, I put together a very simple example of how to write an echo server to get you on your way. Anything received gets echoed back to the client. The server will stay running for 60s. Try telnetting to it on localhost port 6666. Take time to understand exactly what's going on here.

void Main()
    CancellationTokenSource cts = new CancellationTokenSource();
    TcpListener listener = new TcpListener(IPAddress.Any, 6666);
        //just fire and forget. We break from the "forgotten" async loops
        //in AcceptClientsAsync using a CancellationToken from `cts`
        AcceptClientsAsync(listener, cts.Token);
        Thread.Sleep(60000); //block here to hold open the server

async Task AcceptClientsAsync(TcpListener listener, CancellationToken ct)
    var clientCounter = 0;
    while (!ct.IsCancellationRequested)
        TcpClient client = await listener.AcceptTcpClientAsync()
        //once again, just fire and forget, and use the CancellationToken
        //to signal to the "forgotten" async invocation.
        EchoAsync(client, clientCounter, ct);

async Task EchoAsync(TcpClient client,
                     int clientIndex,
                     CancellationToken ct)
    Console.WriteLine("New client ({0}) connected", clientIndex);
    using (client)
        var buf = new byte[4096];
        var stream = client.GetStream();
        while (!ct.IsCancellationRequested)
            //under some circumstances, it's not possible to detect
            //a client disconnecting if there's no data being sent
            //so it's a good idea to give them a timeout to ensure that 
            //we clean them up.
            var timeoutTask = Task.Delay(TimeSpan.FromSeconds(15));
            var amountReadTask = stream.ReadAsync(buf, 0, buf.Length, ct);
            var completedTask = await Task.WhenAny(timeoutTask, amountReadTask)
            if (completedTask == timeoutTask)
                var msg = Encoding.ASCII.GetBytes("Client timed out");
                await stream.WriteAsync(msg, 0, msg.Length);
            //now we know that the amountTask is complete so
            //we can ask for its Result without blocking
            var amountRead = amountReadTask.Result;
            if (amountRead == 0) break; //end of stream.
            await stream.WriteAsync(buf, 0, amountRead, ct)
    Console.WriteLine("Client ({0}) disconnected", clientIndex);

You can use TaskFactory.FromAsync to wrap up Begin / End pairs into async-ready operations.

Stephen Toub has an awaitable Socket on his blog which wraps the more efficient *Async endpoints. I recommend combining this with TPL Dataflow to create a fully async-compatible Socket component.

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