Socket Server

namespace SocketServer
{
    static class Program
    {
        private const char LengthPrefixDelimiter = ';';
        private static AutoResetEvent _flipFlop = new AutoResetEvent(false);

        static void Main(string[] args)
        {
            // create a socket
            Socket listener = new Socket(AddressFamily.InterNetwork,
                SocketType.Stream,
                ProtocolType.Tcp);

            // create a local endpoint
            IPHostEntry ipHostInfo = Dns.Resolve(Dns.GetHostName());
            IPEndPoint localEP = new IPEndPoint(ipHostInfo.AddressList.First(), 22222);

            Console.WriteLine("Local address and port : {0}", localEP);

            // bind and listen
            try
            {
                listener.Bind(localEP);
                listener.Listen(1);

                while (true)
                {
                    Console.WriteLine("Waiting for the next connection...");

                    // accept the next connection
                    listener.BeginAccept(AcceptCallback, listener);

                    _flipFlop.WaitOne();
                }
            }
            catch (Exception e)
            {
                Console.WriteLine(e.ToString());
            }

            listener.Accept();
            listener.Listen(1);

        }

        private static void AcceptCallback(IAsyncResult ar)
        {
            // retrieve the listener, accept
            var listener = (Socket)ar.AsyncState;
            var socket = listener.EndAccept(ar);

            Console.WriteLine("Connected a client.");

            // trigger new listen
            _flipFlop.Set();

            // start receiving
            var state = new StateObject();
            state.Socket = socket;
            socket.BeginReceive(state.Buffer,
                0,
                StateObject.BufferSize,
                0,
                ReceiveCallback,
                state);
        }

        private static void ReceiveCallback(IAsyncResult ar)
        {
            // retrieve the state and socket
            StateObject state = (StateObject)ar.AsyncState;
            Socket socket = state.Socket;

            // Read data from the client socket.
            int read = socket.EndReceive(ar);

            // flag to indicate there's more data coming
            bool allRead = true;

            // Data was read from the client socket.
            if (read > 0)
            {
                // convert result and output
                string chunk = Encoding.UTF8.GetString(state.Buffer, 0, read);
                state.StringBuilder.Append(chunk);

                // here's our small protocol implementation: check the length-prefix
                string messageLengthText = state.StringBuilder.SubstringByDelimiter(LengthPrefixDelimiter);
                if (state.TotalSize == 0)
                {
                    if (!string.IsNullOrEmpty(messageLengthText))
                    {
                        // get length and set total size
                        var messageLength = Convert.ToInt32(messageLengthText);
                        state.TotalSize = messageLength;
                    }
                    else
                    {
                        // if we haven't received the delimiter yet (very unlikely),
                        // simply continue reading
                        allRead = false;
                    }
                }

                // simply check if we've read all bytes
                allRead = allRead && state.StringBuilder.Length - messageLengthText.Length - 1 == state.TotalSize;
            }

            // check if we need to listen again
            if (!allRead)
            {
                // receive again
                socket.BeginReceive(state.Buffer,
                0,
                StateObject.BufferSize,
                0,
                ReceiveCallback,
                state);
            }
            else
            {
                // output anything we've received
                if (state.StringBuilder.Length > 0)
                {
                    // prepare result
                    string result = state.StringBuilder.ToString();
                    result = result.Substring(result.IndexOf(LengthPrefixDelimiter) + 1);

                    // output result on console
                    Console.WriteLine("Received a message: {0}", result);
                }

                socket.Shutdown(SocketShutdown.Both);
                socket.Close();
                Console.WriteLine("Closed client connection.");
            }
        }

        public static string SubstringByDelimiter(this StringBuilder sb, char delimiter)
        {
            StringBuilder result = new StringBuilder(sb.Length);
            for (int i = 0; i < sb.Length; i++)
            {
                if (sb[i] == delimiter)
                {
                    return result.ToString();
                }

                result.Append(sb[i]);
            }

            return string.Empty;
        }
    }

    public class StateObject
    {
        public Socket Socket;
        public StringBuilder StringBuilder = new StringBuilder();
        public const int BufferSize = 10;
        public byte[] Buffer = new byte[BufferSize];
        public int TotalSize;
    }
}