UDP.cs

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

// https://gist.github.com/louis-e/888d5031190408775ad130dde353e0fd#file-udpsocket-cs

namespace UDP
{
    public class UDPSocket
    {
        public Socket _socket;
        private const int bufSize = 8 * 1024;
        private State state = new State();
        private EndPoint epFrom = new IPEndPoint(IPAddress.Any, 0);
        private AsyncCallback? recv = null;

        public class State
        {
            public byte[] buffer = new byte[bufSize];
        }

        public void Server(string address, int port)
        {
            _socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            _socket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.ReuseAddress, true);
            _socket.Bind(new IPEndPoint(IPAddress.Parse(address), port));
            Receive();            
        }

        public void Client(string address, int port)
        {
            _socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            _socket.Connect(IPAddress.Parse(address), port);
            Receive();            
        }

        public void Send(string text)
        {
            byte[] data = Encoding.ASCII.GetBytes(text);
            _socket.BeginSend(data, 0, data.Length, SocketFlags.None, (ar) =>
            {
                State so = (State)ar.AsyncState;
                int bytes = _socket.EndSend(ar);
                Console.WriteLine("SEND: {0}, {1}", bytes, text);
            }, state);
        }

        private void Receive()
        {            
            _socket.BeginReceiveFrom(state.buffer, 0, bufSize, SocketFlags.None, ref epFrom, recv = (ar) =>
            {
                try
                {
                    State so = (State)ar.AsyncState;
                    int bytes = _socket.EndReceiveFrom(ar, ref epFrom);
                    _socket.BeginReceiveFrom(so.buffer, 0, bufSize, SocketFlags.None, ref epFrom, recv, so);
                    Console.WriteLine("RECV: {0}: {1}, {2}", epFrom.ToString(), bytes, Encoding.ASCII.GetString(so.buffer, 0, bytes));
                } catch { }
                
            }, state);
        }
    }
}

namespace UDP
{
    class Program
    {    
        static void Main(string[] args)
        {
            UDPSocket s = new UDPSocket();
            s.Server("127.0.0.1", 27000);

            UDPSocket c = new UDPSocket();
            c.Client("127.0.0.1", 27000);
            // c.Send((char));

            Console.ReadKey();
            s._socket.Close(); //Fixed closing bug (System.ObjectDisposedException)
                               //Bugfix allows to relaunch server
            Console.WriteLine("Closed Server \n Press any key to exit");
            Console.ReadKey();
        }
    }
}