AspNetCore中的Map，MapWhen使用

           app.Map("/second", _app =>
            {
                _app.Use(async (context,next)=>{

                    await context.Response.WriteAsync("second middleware\r\n");
                    await next.Invoke();
                });
            });

            app.MapWhen(context => { return context.Request.Query["aaaa"] == "a"; }, _app => {
                _app.Use(async (context, next) => {
                    await context.Response.WriteAsync("third middleware\r\n");
                    await next.Invoke();
                });
            });