C# WebSocket サーバ/ クライアントのサンプル。サーバ、クライアントそれぞれで、標準入力を受け付けてメッセージを送信します。.NET Core 3 / .NET Framework 向けのコンソールアプリです。

Program_client.cs

// WebSocket Client
using System;
using WebSocket4Net;

// NuGet で WebSocket4Net を追加してください
namespace WebSocketClientTest
{
    class Program
    {
        static void Main(string[] args)
        {
            var ws = new WebSocket("ws://localhost:8000");

            ws.MessageReceived += Ws_MessageReceived;
            ws.DataReceived += Ws_DataReceived;
            ws.Opened += Ws_Opened;
            ws.Closed += Ws_Closed;
            ws.Open();

            while (true)
            {
                var str = Console.ReadLine();
                if (str == "exit")
                {
                    break;
                }

                if (ws.State == WebSocketState.Open)
                {
                    ws.Send(str);
                }
                else
                {
                    Console.WriteLine("{0}:wait...", DateTime.Now.ToString());
                }
            }

            ws.Close();
        }

        private static void Ws_Closed(object sender, EventArgs e)
        {
            Environment.Exit(0);
        }

        private static void Ws_Opened(object sender, EventArgs e)
        {
            Console.WriteLine("Opened");
        }

        private static void Ws_DataReceived(object sender, DataReceivedEventArgs e)
        {
            Console.WriteLine($"{e.Data.Length} bytes received.");
        }

        private static void Ws_MessageReceived(object sender, MessageReceivedEventArgs e)
        {
            Console.WriteLine(e.Message);
        }
    }
}

Program_server.cs

// WebSocket Server
using Fleck;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

// NuGet で Fleck を追加してください
namespace WebSocketServer
{
    class Program
    {
        static object lockObj = new object();
        static List<IWebSocketConnection> sockets = new List<IWebSocketConnection>();
        static void Main(string[] args)
        {
            var server = new Fleck.WebSocketServer("ws://0.0.0.0:8000");
           
            server.Start(socket =>
            {
                socket.OnOpen  = () => OnOpen(socket);
                socket.OnClose = () => OnClose(socket);
                socket.OnMessage = message => OnMessage(socket, message);
            });
            string data = "";
            while(!data.StartsWith("exit"))
            {
                data = Console.ReadLine();
                SendMessageToAll(data);
            }
            foreach (var s in sockets)
            {
                s.Close();
            }
            server.Dispose();
        }

        static async void OnOpen(IWebSocketConnection socket)
        {
            lock (lockObj)
            {
                sockets.Add(socket);
            }
            var info = socket.ConnectionInfo;
            Console.WriteLine($"Open: {info.ClientIpAddress} {info.ClientPort}");
            for (int i = 0; i != 10; ++i)
            {
                await Task.Delay(2000);
                var time = DateTime.Now.ToLongTimeString();
                if (socket.IsAvailable)
                {
                    await socket.Send(time);
                }
            }
        }
        static void OnClose(IWebSocketConnection socket)
        {
            var info = socket.ConnectionInfo;
            Console.WriteLine($"Closed {info.ClientIpAddress} {info.ClientPort}");
        }
        static void OnMessage(IWebSocketConnection socket, string message)
        {
            var info = socket.ConnectionInfo;
            Console.WriteLine($"Received from {info.ClientIpAddress} {info.ClientPort}: {message}");
            SendMessageToAll(message);
        }

        static void SendMessageToAll(string message)
        {
            lock (lockObj)
            {
                // remove unused sockets
                List<IWebSocketConnection> soc = new List<IWebSocketConnection>();
                foreach (var s in sockets)
                {
                    if (s.IsAvailable)
                    {
                        soc.Add(s);
                    }
                    sockets = soc;
                }

                // send message
                foreach (var s in sockets)
                {
                    s.Send(message);
                }
            }
        }
    }
}