public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .UseStartup<Startup>();


        public static IWebHostBuilder CreateWebHostBuilder(string[] args)
        {
            return WebHost.CreateDefaultBuilder(args).UseStartup<Startup>();
        }

  /*
             写法1
             */
            app.Run(
                async (context)
                =>
                {
                    await context.Response.WriteAsync("Hello World!_001");
                }
            );
            /*
             写法 2
             */
            test(app);

            app.Run(
                 (context)     // 类似于匿名函数
                 =>
                 {
                     return context.Response.WriteAsync("Hello World!");
                 }