Startup.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.WebSockets;
using System.Runtime.Serialization.Formatters.Binary;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Newtonsoft.Json;

namespace web_sockets_playground
{
    public static class Utils {
        public static UInt16[] GetNumbers(int n) {
            // var r = new Random(0);
            return Enumerable.Range(0, n).Select(x => (System.UInt16)(x % 2 == 0 ? 'a' : 'b')).ToArray();
        }
    }

    public class Startup
    {
        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        public IConfiguration Configuration { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.Configure<CookiePolicyOptions>(options =>
            {
                // This lambda determines whether user consent for non-essential cookies is needed for a given request.
                options.CheckConsentNeeded = context => true;
                options.MinimumSameSitePolicy = SameSiteMode.None;
            });


            services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
        }

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

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseCookiePolicy();
            var webSocketOptions = new WebSocketOptions()
            {
                KeepAliveInterval = TimeSpan.FromSeconds(120),
                ReceiveBufferSize = 4 * 1024
            };
            app.UseWebSockets(webSocketOptions);

            app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });

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

            });
        }

        private async Task NumbersResponder(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)
            {
                var br = new BinaryReader(new MemoryStream(buffer));
                var format = br.ReadInt32();
                var requestSize = br.ReadInt32();
                // Console.WriteLine($"Request {format} (binary socket) {requestSize}");

                var numbers = Utils.GetNumbers(requestSize);

                using(var ms = new MemoryStream()) {
                    if (format == 0) { // binary
                        using(var bw = new BinaryWriter(ms)) {
                            bw.Write(numbers.Length);
                            foreach (var x in numbers) {
                                bw.Write(x);
                            }

                            bw.Flush();
                            ms.Seek(0, SeekOrigin.Begin);
                            buffer = ms.ToArray();
                            await webSocket.SendAsync(buffer, WebSocketMessageType.Binary, true, CancellationToken.None);
                        }
                    } else { // JSON
                        using (var sw = new StreamWriter(ms)) {
                            var s = JsonConvert.SerializeObject(numbers);
                            sw.Write(s);
                            sw.Flush();
                            ms.Seek(0, SeekOrigin.Begin);
                            buffer = ms.ToArray();
                            await webSocket.SendAsync(buffer, WebSocketMessageType.Binary, true, CancellationToken.None);
                        }
                    }
                }

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

    }
}