Startup.cs

using System;
using System.Collections.Generic;
using System.Net.Http;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Cors.Infrastructure;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using TodoWebApi.Data;

namespace TodoWebApi
{
	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)
		{
			var todoRepository = new TodoWebApi.Data.InMemory.TodoRepository();

			services.AddSingleton(typeof(ITodoRepository), todoRepository);
			services.AddMvc()
					.AddJsonOptions(opt => {
						opt.SerializerSettings.NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore;
					});
		}

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

			app.UseCors((CorsPolicyBuilder corsBuilder) =>
			{
				corsBuilder.AllowAnyOrigin();
				corsBuilder.AllowAnyMethod();
				corsBuilder.AllowAnyHeader();
				corsBuilder.Build();
			});

			app.MapWhen(context => context.Request.Method == HttpMethod.Options.ToString(), OptionsMiddleware);

			app.UseMvc();
		}

		private static void OptionsMiddleware(IApplicationBuilder app)
		{
			app.Run(async context =>
			{
				await context.Response.WriteAsync("");
			});
		}
	}
}