740092 を簡潔にした感じ。

MulticastingSample.cs

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;


namespace demo.multicasting {


    /// <summary>
    /// 
    /// </summary>
    class MulticastingSample {
        /// <summary>
        /// 
        /// </summary>
        void run() {
            UdpClient client = new UdpClient( 2000, AddressFamily.InterNetwork );
            IPAddress group_address = IPAddress.Parse( "239.0.0.0" );

            IPEndPoint remote_point = new IPEndPoint( group_address, 1000 );

            client.JoinMulticastGroup( group_address );

            Thread there_worker = new Thread( multicastConversion );
            there_worker.Start();

            Thread.Sleep( 2000 );

            Console.WriteLine( "Alice:" );
            string[] greetings = new string[] { "Hello, Bob.", "Have a nice day.", "Over" };
            sendGreetings( client, greetings, remote_point );

            string ret = receiveUntilStop( client, greetings[2] );
            Console.WriteLine( "Alice received:\n{0}\n", ret );

            there_worker.Abort();

            client.DropMulticastGroup( group_address );
        }


        /// <summary>
        /// 
        /// </summary>
        void multicastConversion() {
            UdpClient client = new UdpClient( 1000, AddressFamily.InterNetwork );
            IPAddress group_address = IPAddress.Parse( "239.0.0.0" );

            client.JoinMulticastGroup( group_address );

            IPEndPoint remote_point = new IPEndPoint( group_address, 2000 );

            string ret = receiveUntilStop( client, "Over" );
            Console.WriteLine( "Bob received: \n{0}\n", ret );

            Thread.Sleep( 2000 );

            Console.WriteLine( "\nBob:" );
            string[] greetings = new string[] { "Hello, Alice.", "You too.", "Over" };
            sendGreetings( client, greetings, remote_point );
        }


        /// <summary>
        /// 
        /// </summary>
        /// <param name="client"></param>
        /// <param name="terminator_segment"></param>
        /// <returns></returns>
        string receiveUntilStop(UdpClient client, string terminator_segment) {
            StringBuilder result_builder = new StringBuilder();
            string segment = string.Empty;
            Encoding encoding = Encoding.ASCII;

            IPEndPoint remote_point = new IPEndPoint( IPAddress.Any, 50 );

            while ( !segment.StartsWith( terminator_segment ) ) {
                byte[] received_data = client.Receive( ref remote_point );

                segment = encoding.GetString( received_data );
                result_builder.AppendLine( segment );
            }
            return result_builder.ToString();
        }


        /// <summary>
        /// 
        /// </summary>
        /// <param name="client"></param>
        /// <param name="greetings"></param>
        /// <returns></returns>
        bool sendGreetings(UdpClient client, string[] greetings, IPEndPoint remote_point) {
            int count = 0;
            byte[] send_bytes = null;

            foreach ( string greeting in greetings ) {
                Console.WriteLine( "    send to: \"{0}\"", greeting );
                send_bytes = getByteArray( greeting );
                client.Send( send_bytes, send_bytes.Length, remote_point );

                if ( count <= greetings.Length )
                    Thread.Sleep( 1000 );

                ++count;
            }
            return true;
        }


        /// <summary>
        /// 
        /// </summary>
        /// <param name="s"></param>
        /// <returns></returns>
        byte[] getByteArray(string s) {
            return Encoding.UTF8.GetBytes( s );
        }


        /// <summary>
        /// 
        /// </summary>
        static void Main() {
            MulticastingSample progn = new MulticastingSample();

            progn.run();
        }
    }


}