UDP client isn't receiving a response

Question

I am creating a solution for data transfer to and from a python script and WinForm application. My current problem is that when the data is received and a received message is generated and sent back, the python script never sees it.

It was working before I introduced async functionality, but I wanted to use that to shut down the server/thread that was handling the server.

here is my server code (C#):

private void btnConnect_Click(object sender, EventArgs e)
        {
            InitializeUDP();
        }

        public void InitializeUDP()
        {
            //Create the server
            endPoint = new IPEndPoint(IPAddress.Any, PORT);
            udpClient = new UdpClient(endPoint);

            ShowMsg("Waiting for a client...");
            //Create the client end.

            //Start listening
            listenThread = new Thread(new ThreadStart(Listening));
            listenThread.Start();
        }
        private void Listening()
        {
            byte[] resp;

            //Listening loop
            Task.Run(async () =>
            {
                while (true)
                {
                    //Receive a message from a client
                    var data = await udpClient.ReceiveAsync();
                    string receivedMsg = Encoding.ASCII.GetString(data.Buffer);
                    //Show the message.
                    this.Invoke(new ShowMessageMethod(ShowMsg), new object[] { "Client:" + receivedMsg });
                    //Send a response message
                    resp = Encoding.ASCII.GetBytes("Server:" + receivedMsg);
                    udpClient.Send(resp, resp.Length, endPoint);
                    //Sleep for UI to work
                    Thread.Sleep(500);
                }
            });
        }
        private void ShowMsg(string msg)
        {
            this.tbTest.Invoke((MethodInvoker)(() => this.tbTest.Text += msg + "\r\n"));
        }

        private void btnDisconnect_Click(object sender, EventArgs e)
        {
            Stop();
        }
        private void Stop()
        {
            //Stop listening
            ShowMsg("Server stopping...");
            udpClient.Close();
            ShowMsg("Server stopped.");
        }

here is my client code (python):

import socket

msgFromClient       = "Hello UDP Server"
bytesToSend         = str.encode(msgFromClient)
serverAddressPort   = ("127.0.0.1", 12345)
bufferSize          = 1024

# Create a UDP socket at client side

UDPClientSocket = socket.socket(family=socket.AF_INET, type=socket.SOCK_DGRAM)

# Send to server using created UDP socket

UDPClientSocket.sendto(bytesToSend, serverAddressPort)

msgFromServer = UDPClientSocket.recvfrom(bufferSize)

msg = "Message from Server {}".format(msgFromServer[0])

print(msg)

Answer 1

You're sending a packet to address IPAddress.Any and the server's port, instead of the client's IP address and port.

The client's IP address and port are stored in data.RemoteEndPoint .

How to find this: notice that ReceiveAsync returns a Task<UdpReceiveResult> - UdpReceiveResult is the "return type" of the task - await waits for the task to complete and then gives you the UdpReceiveResult . Then see that UdpReceiveResult has a field called RemoteEndPoint which is what you want.