Startup.cs

#define UseOptions // or NoOptions or UseOptionsAO
using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.WebSockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Logging.Console;
using Microsoft.Extensions.Logging.Debug;

namespace EchoApp
{
    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit http://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddLogging(builder =>
            {
                builder.AddConsole()
                    .AddDebug()
                    .AddFilter<ConsoleLoggerProvider>(category: null, level: LogLevel.Debug)
                    .AddFilter<DebugLoggerProvider>(category: null, level: LogLevel.Debug);
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

#if NoOptions
            #region UseWebSockets
            app.UseWebSockets();
            #endregion
#endif
#if UseOptions
            #region UseWebSocketsOptions
            var webSocketOptions = new WebSocketOptions() 
            {
                KeepAliveInterval = TimeSpan.FromSeconds(120),
                ReceiveBufferSize = 4 * 1024
            };

            app.UseWebSockets(webSocketOptions);
            #endregion
#endif

#if UseOptionsAO
            #region UseWebSocketsOptionsAO
            var webSocketOptions = new WebSocketOptions()
            {
                KeepAliveInterval = TimeSpan.FromSeconds(120),
                ReceiveBufferSize = 4 * 1024
            };
            webSocketOptions.AllowedOrigins.Add("https://client.com");
            webSocketOptions.AllowedOrigins.Add("https://www.client.com");

            app.UseWebSockets(webSocketOptions);
            #endregion
#endif

            #region AcceptWebSocket
            app.Use(async (context, next) =>
            {
                if (context.Request.Path == "/ws")
                {
                    if (context.WebSockets.IsWebSocketRequest)
                    {
                        WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
                        await Echo(context, webSocket);
                    }
                    else
                    {
                        context.Response.StatusCode = 400;
                    }
                }
                else
                {
                    await next();
                }

            });
#endregion
            app.UseFileServer();
        }
#region Echo
        private async Task Echo(HttpContext context, WebSocket webSocket)
        {
            var buffer = new byte[1024 * 4];
            WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
            while (!result.CloseStatus.HasValue)
            {
                await webSocket.SendAsync(new ArraySegment<byte>(buffer, 0, result.Count), result.MessageType, result.EndOfMessage, CancellationToken.None);

                result = await webSocket.ReceiveAsync(new ArraySegment<byte>(buffer), CancellationToken.None);
            }
            await webSocket.CloseAsync(result.CloseStatus.Value, result.CloseStatusDescription, CancellationToken.None);
        }
#endregion
    }
}