using System;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace send {
    public partial class Form1 : Form {

        public Form1() {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e) {

        }

        private async void button1_Click(object sender, EventArgs e) { // async追加

            // https://dobon.net/vb/dotnet/internet/udpclient.html
            string remoteHost = "192.168.3.166"; //192.168.3.255 だと3セグメント全てに送信(ブロードキャスト)
            int remotePort = 2002;

            //UdpClientオブジェクトを作成する
            System.Net.Sockets.UdpClient udp = new System.Net.Sockets.UdpClient();
            long x = 0;

            await Task.Run(() => {

                for (; ; )               {
                    //送信するデータを作成する
                    Thread.Sleep(500);

                    string sendMsg = x.ToString();
                    x++;
                    byte[] sendBytes = System.Text.Encoding.UTF8.GetBytes(sendMsg);

                    //リモートホストを指定してデータを送信する
                    udp.Send(sendBytes, sendBytes.Length, remoteHost, remotePort);
                }

            });
        }
    }
}

--------------------------------------------------------------------------------------------------
データグラム受信側

using System;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace rcv {
    public partial class Form1 : Form {
        public Form1() {
            InitializeComponent();
        }

        private async void Form1_Load(object sender, EventArgs e) {


            // https://dobon.net/vb/dotnet/internet/udpclient.html
            string localIpString = "192.168.3.166";
            System.Net.IPAddress localAddress = System.Net.IPAddress.Parse(localIpString);
            int localPort = 2002;// 2002;

            //UdpClientを作成し、ローカルエンドポイントにバインドする
            System.Net.IPEndPoint localEP = new System.Net.IPEndPoint(localAddress, localPort);
            System.Net.Sockets.UdpClient udp = new System.Net.Sockets.UdpClient(localEP);

            string rcvMsg;
            byte[] rcvBytes;
            System.Net.IPEndPoint remoteEP = null;

            for (; ; ) {
                await Task.Run(() => {
                   
                    rcvBytes = udp.Receive(ref remoteEP);
                    rcvMsg = System.Text.Encoding.UTF8.GetString(rcvBytes);

                    this.Invoke(new Action(() => {
                        textBox1.Text = rcvMsg;
                    }));
                });
            }
        }
    }
}