C＃异步和等待流

Question

I am trying to understand async and await flow . Started looking at code from
John Atten's blog about Owin and Katana . While trying to look at the execution steps I found few steps in the flow (Steps 9 and 16), which I am not able to understand as to why the execution will traverse.

code:

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Microsoft.Owin;
using Microsoft.Owin.Hosting;
using Owin;
namespace KatanaConsole
{
    //use an alias for OWIN APPFunc
    using AppFunc= Func<IDictionary<string,object>,Task>;
    class Program
    {
        static void Main(string[] args)
        {

            var uri = "http://localhost:8080";

            using ( WebApp.Start<StartUp>(uri))
            {
                Console.WriteLine("Web Server started on port 2323");
                 Console.WriteLine("Server Started; Press enter to Quit");
                Console.ReadLine();
            }
        }
    }

    public class StartUp
    {
        public void Configuration(IAppBuilder appBuilder)
        {
        var firstMiddleware= new Func<AppFunc,AppFunc>(FirstMiddleware);
        var secondMiddleware = new Func<AppFunc, AppFunc>(SecondMiddleware);

        appBuilder.Use(firstMiddleware);
        appBuilder.Use(secondMiddleware);

        }


        public AppFunc FirstMiddleware(AppFunc next)
        {

            AppFunc appFunc = async environment =>
            {
                IOwinContext context = new OwinContext(environment);
                await context.Response.WriteAsync("<h1> Middleware:1 ->  Hello from X1</h1>");
                await next.Invoke(environment);
            };
            return appFunc;
        }

        public AppFunc SecondMiddleware(AppFunc next)
        {

            AppFunc appFunc = async (IDictionary<string, object> environment) =>
            {
                IOwinContext context = new OwinContext(environment);
                await context.Response.WriteAsync("<h1> Middleware:2 ->  Hello from X2</h1>");
                await next.Invoke(environment);
            };
            return appFunc;
        }
    }


}

The code flow when I try to access localhost:8080

Flow:

Enters ->First Middleware

1. IOwinContext context = new OwinContext(environment);
2. await context.Response.WriteAsync("<h1> Middleware:1 -> Hello from X1</h1>"); // Response Sent to browser
3. await next.Invoke(environment);

Enters -> Second Middleware

4. IOwinContext context = new OwinContext(environment);

5. await context.Response.WriteAsync("<h1> Middleware:2 -> Hello from X2</h1>");// Response Sent to browser

6. await next.Invoke(environment);

Enters (Back to Calling Method) -> First Middleware

7. await next.Invoke(environment);

8. Exits -> FirstMiddleWare

Re-Enters ->First Middleware

9. IOwinContext context = new OwinContext(environment);

10. await context.Response.WriteAsync("<h1> Middleware:1 -> Hello from X1</h1>"); // No Response Sent to browser

11. await next.Invoke(environment);

Re-Enters -> Second Middleware

12. IOwinContext context = new OwinContext(environment);

13. await context.Response.WriteAsync("<h1> Middleware:2 -> Hello from X2</h1>");// No Response Sent to browser

14. await next.Invoke(environment);

Re-Enters (Back To Calling Method) -> First Middleware

15. await next.Invoke(environment);

16. Exits -> FirstMiddleWare

Execution Stops My question is why does it traverse again Steps 9 and 16?

and add to it, even though Step 9 and 16 are traversed , response does not change. So I am guessing it is doing some post condition check there.

** Edit - 1 **

Added All steps traversed to make it more clear( added steps 9 to 16)

Answer 1

Steps 9 to 16 are traversed because of an automatic second request you get from the browser for the favicon . If you pay attention to the context.Request.Path you will notice that the first round through the middleware is for "/" and the second for "/favicon.ico". Actually the behavior depends on the browser you are using to make the request. I've tested with the most resent version of a couple of browsers and with Webkit browsers (Chrome and Opera) plus IE i always get a request for the favicon. With FF i do get it just the first time and it's being cached. And with Edge - it's not been requested at all.