props/MultiShop/server/Startup.cs

using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.EntityFrameworkCore;
using MultiShop.Data;
using MultiShop.Models;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace MultiShop
{
    public class Startup
    {
        public Startup(IConfiguration configuration, IHostEnvironment environment)
        {
            Configuration = configuration;
            Environment = environment;
        }

        public IConfiguration Configuration { get; }
        public IHostEnvironment Environment { get; }

        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<ApplicationDbContext>(options =>
                options.UseSqlite(
                    Configuration.GetConnectionString("DefaultConnection")));

            services.AddDatabaseDeveloperPageExceptionFilter();

            services.AddDefaultIdentity<ApplicationUser>(options => options.SignIn.RequireConfirmedAccount = true)
                .AddEntityFrameworkStores<ApplicationDbContext>();

            services.AddIdentityServer()
                .AddApiAuthorization<ApplicationUser, ApplicationDbContext>(
                    options => {
                        options.Clients.AddIdentityServerSPA("MultiShop", spa => {
                            spa.WithRedirectUri("/authentication/silent-login-callback.html");
                            spa.WithRedirectUri("/authentication/login-callback");
                            spa.WithLogoutRedirectUri("/authentication/logout-callback");
                        });
                    });

            services.AddAuthentication()
                .AddIdentityServerJwt();
            services.AddControllersWithViews();
            services.AddRazorPages();
            services.AddSpaStaticFiles(configuration =>
            {
                configuration.RootPath = "client/dist";
            });
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
                app.UseMigrationsEndPoint();
            }
            else
            {
                app.UseExceptionHandler("/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            if (!env.IsDevelopment()) app.UseHttpsRedirection();
            app.UseStaticFiles();
            if (!env.IsDevelopment()) app.UseSpaStaticFiles();

            app.UseRouting();

            app.UseAuthentication();
            app.UseIdentityServer();
            app.UseAuthorization();
            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllerRoute(
                    name: "default",
                    pattern: "{controller}/{action=Index}/{id?}");
                endpoints.MapRazorPages();
            });

            app.UseSpa(spa =>
            {
                spa.Options.SourcePath = "../client"; // "May not exist in published applications" - https://docs.microsoft.com/en-us/dotnet/api/microsoft.aspnetcore.spaservices.spaoptions.sourcepath?view=aspnetcore-5.0#Microsoft_AspNetCore_SpaServices_SpaOptions_SourcePath 

                if (env.IsDevelopment())
                {
                    spa.UseProxyToSpaDevelopmentServer(Configuration["SPA:BaseUri"]);
                }
            });
        }
    }
}