...
_serverSocket.Listen(4);
_serverSocket.BeginAccept(new AsyncCallback(OnAccept), null);
...

private void OnAccept(IAsyncResult ar) {
   try {
      Socket clientSocket = _serverSocket.EndAccept(ar);
      _serverSocket.BeginAccept(new AsyncCallback(OnAccept), null);
      clientSocket.BeginReceive(byteData, 0, byteData.Length, SocketFlags.None, new AsyncCallback(OnReceive), clientSocket);
   }
   catch (Exception ex) {
      AppendMessage(ex.Message, Messages.Exception);
   }
}

private void OnReceive(IAsyncResult ar) {
   try {
      Socket clientSocket = (Socket)ar.AsyncState;
      clientSocket.EndReceive(ar);
   ...
}