C#

class Program
  {
      public static bool listen = true;
      public static IPEndPoint endPoint;
      public static UdpClient udpClient;

      static void Main(string[] args)
      {
          //Example of crummy server app running
          //Starts the listener, and writes the received data to the console
          //while allowing the user to interact with program - "q" quits.
          Console.WriteLine("Server Running...");


          endPoint = new IPEndPoint(IPAddress.Any, 69);
          udpClient = new UdpClient(endPoint);

          ReceiveMessages();
          while (listen)
          {
              string str = Console.ReadLine();
              if (str == "q")
              {
                  listen = false;
              }
              else
              {
                  Console.WriteLine(str);
              }
          }
      }

      //This is the callback, used to process the received data
      public static void ReceiveCallback(IAsyncResult ar)
      {
          UdpClient u = (UdpClient)((UdpState)(ar.AsyncState)).udpClient;
          IPEndPoint e = (IPEndPoint)((UdpState)(ar.AsyncState)).endPoint;

          //End the receive to retrieve the data
          Byte[] receiveBytes = u.EndReceive(ar, ref e);

          //Do some processing
          Console.WriteLine(Encoding.ASCII.GetString(receiveBytes) + "\r\n");

          //Start listening again...
          ReceiveMessages();

      }

      //This listens to the port and calls the Async Callback when data is retrieved
      public static void ReceiveMessages()
      {
          UdpState state = new UdpState();
          state.endPoint = endPoint;
          state.udpClient = udpClient;
          udpClient.BeginReceive(new AsyncCallback(ReceiveCallback), state);

      }
  }

  //UdpState object - records the state of the of the UdpClient
  class UdpState
  {
      public UdpClient udpClient;
      public IPEndPoint endPoint;
  }

<br />
UdpClient u = (UdpClient)((UdpState)(ar.AsyncState)).udpClient;<br />
IPEndPoint e = (IPEndPoint)((UdpState)(ar.AsyncState)).endPoint;<br />