Simple C# UDP server/client in 56 lines

UDPSocket.cs

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

namespace UDP
{
    public class UDPSocket
    {
        private Socket _socket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
        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.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.ReuseAddress, true);
            _socket.Bind(new IPEndPoint(IPAddress.Parse(address), port));
            Receive();            
        }

        public void Client(string address, int port)
        {
            _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) =>
            {
                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));
            }, state);
        }
    }
}

Usage.cs

using System;

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("TEST!");

            Console.ReadKey();
        }
    }
}

Is there a way to send from server back to client?

Declare a client on the servers code and connect him to the loopback ip address.

Can you give an example ^^

You basically just have to declare a new client (in addition to the server and the already existing clients):
UDPSocket c = new UDPSocket();
Then you connect your just new declared client to your server (I assumed that the server runs on localhost and port 27000)
c.Client("127.0.0.1", 27000);
After that you can send your message over this new declared client to the other clients.
c.Send("TEST!");

Important Note: Using UDP over Internet

I used this code as part of a multiplayer game server component. It worked fine once I added a SendTo() method for the server sending to clients. However, once I moved from testing on LAN to over the Internet I had issues. I had a static IP for my router and set up port forwarding. Was receiving UDP packets from clients, but any responses back to clients were disappearing. However, I removed the line:

_socket.SetSocketOption(SocketOptionLevel.IP, SocketOptionName.ReuseAddress, true);

And it started working over Internet too.

This works for me locally but when I send to a remote host I don't receive anything even though Wireshark shows the data coming back correctly from the remote host.

UDPSocket s = new UDPSocket();
s.Server("127.0.0.1", 2000);//binds to the local endpoint

UDPSocket c = new UDPSocket();
c.Client("192.168.26.186", 1999);//connects to the remote host

c.Send("0500020a04");

Summary from Wireshark:

Any ideas why?

Assuming your remote host is in your local network; You have to use your local ip address (192.168.x.x) instead of the localhost 127.0.0.1 in your server declaration line (s.Server("127.0.0.1", 2000);). In addition to that you also have to use the same port for both server and client (e.g. 2000) 👍

Assuming your remote host is in your local network; You have to use your local ip address (192.168.x.x) instead of the localhost 127.0.0.1 in your server declaration line (s.Server("127.0.0.1", 2000);). In addition to that you also have to use the same port for both server and client (e.g. 2000) 👍

Thanks for responding louis-e.

The remote host is in the local network so I am working with local Server 192.168.30.200 and remote Client 192.168.26.186. The client is a piece of industrial hardware that only receives on port 1999 and sends to port 2000 so I can't use the same port number. I proved correct communication using Packet Sender.

Perhaps I am going about this wrong. I am sending UDP successfully. As you can see from the Wireshark screen shot, the device is responding correctly to port 2000 of the local Server. I just am not receiving the data.

What is a better approach to receive the UDP from port 2000?

Okay, I understand. Did you try to use s.Server("192.168.30.200", 2000); for declaring the server?

That was it. I thought I has tried that but I guess not. It works now. Thanks so much!

Thanks a lot! , It works fluently in my project

Thank you. Can you do it for TCP?

s.Server("127.0.0.1", 27000);
It's a local IP or remote? What if I don't know a remote IP? I want to receive from any socket.

Thanks! I was looking for a good example of UDP and this is a nice starting point.

Is it suppose to work easily ? I'm new with that client/servers code.

How to send a full file or image that sized 0.6mb or 600kb