Startup.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Authentication.Cookies;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.HttpsPolicy;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Microsoft.OpenApi.Models;

using TodoApi.Models;
using TodoApi.Services;

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

        public IConfiguration Configuration { get; }

        readonly string LocalDevOrigins = "_localDevOrigins";
        // This method gets called by the runtime. Use this method to add services to the container.
        public void ConfigureServices(IServiceCollection services)
        {

            services.AddCors(options =>
            {
                options.AddPolicy(name: LocalDevOrigins,
                    builder => 
                    {
                        builder.WithOrigins( "http://localhost:3000")
                        .AllowAnyHeader()
                        .AllowAnyMethod()
                        .AllowCredentials();
                    });
            });

            services.Configure<MongoDatabaseSettings>(
                Configuration.GetSection(nameof(MongoDatabaseSettings)));
        
            services.AddSingleton<IMongoDatabaseSettings>(sp =>
                sp.GetRequiredService<IOptions<MongoDatabaseSettings>>().Value);

            services.AddSingleton<UserService>();

            services.AddAuthentication(CookieAuthenticationDefaults.AuthenticationScheme)
                .AddCookie(options => 
                {

                });

            services.AddControllers()
                .AddNewtonsoftJson(options => options.UseMemberCasing());

            services.AddSwaggerGen(c =>
            {
                c.SwaggerDoc("v1", new OpenApiInfo { Title = "TodoApi", Version = "v1" });
            });
        }

        // 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.UseSwagger();
                app.UseSwaggerUI(c => c.SwaggerEndpoint("/swagger/v1/swagger.json", "TodoApi v1"));
                
            } else {
                app.UseHttpsRedirection();

            }

            app.UseRouting();
            app.UseCors(LocalDevOrigins);
            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapControllers();
            });
        }
    }
}