Server myServer = new Server();
myServer.Start();

using System.Net.Sockets;
 
public class Server
{
  private static Socket listener;
  public static ManualResetEvent allDone = new ManualResetEvent(false);
  public const int _bufferSize = 1024;
  public const int _port = 50000;
  public static bool _isRunning = true;
 
  class StateObject
  {
    public Socket workSocket = null;
    public byte[] buffer = new byte[bufferSize];
    public StringBuilder sb = new StringBuilder();
  }
 
  // Returns the string between str1 and str2
  static string Between(string str, string str1, string str2)
  {
    int i1 = 0, i2 = 0;
    string rtn = "";
 
    i1 = str.IndexOf(str1, StringComparison.InvariantCultureIgnoreCase);
    if (i1 > -1)
    {
      i2 = str.IndexOf(str2, i1 + 1, StringComparison.InvariantCultureIgnoreCase);
      if (i2 > -1)
      {
        rtn = str.Substring(i1 + str1.Length, i2 - i1 - str1.Length);
      }
    }
    return rtn;
  }
 
  // Checks if the socket is connected
  static bool IsSocketConnected(Socket s)
  {
    return !((s.Poll(1000, SelectMode.SelectRead) && (s.Available == 0)) || !s.Connected);
  }
 
  // Insert all the other methods here.
}

public void Start()
{
  IPHostEntry ipHostInfo = Dns.GetHostEntry(Dns.GetHostName());
  IPEndPoint localEP = new IPEndPoint(IPAddress.Any, _port);
  listener = new Socket(localEP.Address.AddressFamily, SocketType.Stream, ProtocolType.Tcp);
  listener.Bind(localEP);
 
  while (_IsRunning)
  {
    allDone.Reset();
    listener.Listen(10);
    listener.BeginAccept(new AsyncCallback(acceptCallback), listener);
    bool isRequest = allDone.WaitOne(new TimeSpan(12, 0, 0)); // Blocks for 12 hours
 
    if (!isRequest)
    {
      allDone.Set();
      // Do some work here every 12 hours
    }
  }
  listener.Close();
}

static void acceptCallback(IAsyncResult ar)
{
  // Get the listener that handles the client request.
  Socket listener = (Socket)ar.AsyncState;
 
  if (listener != null)
  {
    Socket handler = listener.EndAccept(ar);
 
    // Signal main thread to continue
    allDone.Set();
 
    // Create state
    StateObject state = new StateObject();
    state.workSocket = handler;
    handler.BeginReceive(state.buffer, 0, _bufferSize, 0, new AsyncCallback(readCallback), state);
  }
}

static void readCallback(IAsyncResult ar)
{
  StateObject state = (StateObject)ar.AsyncState;
  Socket handler = state.workSocket;
 
  if (!IsSocketConnected(handler)) 
  {
    handler.Close();
    return;
  }
 
  int read = handler.EndReceive(ar);
 
  // Data was read from the client socket.
  if (read > 0)
  {
    state.sb.Append(Encoding.UTF8.GetString(state.buffer, 0, read));
 
    if (state.sb.ToString().Contains("<!--ENDSOCKET-->"))
    {
      string toSend = "";
      string cmd = ts.Strings.Between(state.sb.ToString(), "<!--SOCKET-->", "<!--ENDSOCKET-->");
           
      switch (cmd)
      {
        case "Hi!":
          toSend = "How are you?";
          break;
        case "Milky Way?":
          toSend = "No I am not.";
          break;
      }
 
      toSend = "<!--RESPONSE-->" + toSend + "<!--ENDRESPONSE-->";
 
      byte[] bytesToSend = Encoding.UTF8.GetBytes(toSend);
      handler.BeginSend(bytesToSend, 0, bytesToSend.Length, SocketFlags.None
        , new AsyncCallback(sendCallback), state);
    }
    else 
    {
      handler.BeginReceive(state.buffer, 0, _bufferSize, 0
          , new AsyncCallback(readCallback), state);
    }
  }
  else
  {
      handler.Close();
  }
}

static void sendCallback(IAsyncResult ar)
{
  StateObject state = (StateObject)ar.AsyncState;
  Socket handler = state.workSocket;
  handler.EndSend(ar);
 
  StateObject newstate = new StateObject();
  newstate.workSocket = handler;
  handler.BeginReceive(newstate.buffer, 0, StateObject.BufferSize, 0, new AsyncCallback(readCallback), newstate);
}