Challenge authentication using a custom middleware (no mvc).

Startup.cs

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using System.IdentityModel.Tokens.Jwt;

namespace AspNetCoreAuthTest
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            JwtSecurityTokenHandler.DefaultInboundClaimTypeMap.Clear();

            // Configure authentication providers.
            services
                .AddAuthentication(options =>
                {
                    options.DefaultScheme = "Cookies";
                    options.DefaultChallengeScheme = "facebook";
                })
                .AddCookie("Cookies")
                .AddFacebook("facebook", options =>
                {
                    // Set your own app id and secret here.
                    options.AppId = "xxx";
                    options.AppSecret = "xxx";
                });

        }

        public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        {
            app.UseDeveloperExceptionPage();

            app.UseAuthentication();

            app.Use(async (context, next) =>
            {
                if (context.User.Identity.IsAuthenticated)
                {
                    await next.Invoke();
                }
                else
                {
                    // Challenge authentication if the user is not authenticated.
                    await context.ChallengeAsync();
                }
            });

            app.Run(async (context) =>
            {
                await context.Response.WriteAsync("Hello World!");
            });
        }
    }
}