class AsynchronousIoServer
{
    private Socket _serverSocket;
    private int _port;
    public AsynchronousIoServer(int port_num)
    {
        _port = port_num;
    }
    private class ConnectionInfo
    {
        public Socket Socket;
        public byte[] Buffer;
    }
    private List<ConnectionInfo> _connections =
    new List<ConnectionInfo>();
    public void Start()
    {
        SetupServerSocket();
        for (int i = 0; i < 10; i++)
            _serverSocket.BeginAccept(
            new AsyncCallback(AcceptCallback), _serverSocket);
    }
    private void SetupServerSocket()
    {
        IPHostEntry localMachineInfo =
        Dns.GetHostEntry(Dns.GetHostName());
        IPEndPoint myEndpoint = new IPEndPoint(
        localMachineInfo.AddressList[0], _port);
        _serverSocket = new Socket(myEndpoint.Address.AddressFamily,
        SocketType.Stream, ProtocolType.Tcp);
        _serverSocket.Bind(myEndpoint);
        _serverSocket.Listen((int)SocketOptionName.MaxConnections);
    }
    private void AcceptCallback(IAsyncResult result)
    {
        ConnectionInfo connection = new ConnectionInfo();
        try
        {
            Socket s = (Socket)result.AsyncState;
            connection.Socket = s.EndAccept(result);
            connection.Buffer = new byte[255];
            lock (_connections) _connections.Add(connection);
            connection.Socket.BeginReceive(connection.Buffer, 0,
            connection.Buffer.Length, SocketFlags.None,
            new AsyncCallback(ReceiveCallback), connection);
            _serverSocket.BeginAccept(new AsyncCallback(AcceptCallback),
            result.AsyncState);
        }
        catch (SocketException exc)
        {
            CloseConnection(connection);
            Console.WriteLine("Socket exception: " + exc.SocketErrorCode);
        }
        catch (Exception exc)
        {
            CloseConnection(connection);
            Console.WriteLine("Exception: " + exc);
        }
    }
    private void ReceiveCallback(IAsyncResult result)
    {
        ConnectionInfo connection = (ConnectionInfo)result.AsyncState;
        try
        {
            int bytesRead = connection.Socket.EndReceive(result);
            if (0 != bytesRead)
            {
                lock (_connections)
                {
                    foreach (ConnectionInfo conn in _connections)
                    {
                        if (connection != conn)
                        {
                            conn.Socket.Send(connection.Buffer, bytesRead,
                            SocketFlags.None);
                        }
                    }
                }
                connection.Socket.BeginReceive(connection.Buffer, 0,
                connection.Buffer.Length, SocketFlags.None,
                new AsyncCallback(ReceiveCallback), connection);
            }
            else CloseConnection(connection);
        }
        catch (SocketException exc)
        {
            CloseConnection(connection);
            Console.WriteLine("Socket exception: " + exc.SocketErrorCode);
        }
        catch (Exception exc)
        {
            CloseConnection(connection);
            Console.WriteLine("Exception: " + exc);
        }
    }
    private void CloseConnection(ConnectionInfo ci)
    {
        ci.Socket.Close();
        lock (_connections) _connections.Remove(ci);
    }
}