how to make URL in MVC 5 SEO Friendly and Consistent?

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ASP.NET

routing

MVC5

I am trying to make my websites urls more SEO friendly, so I Used This approach to change routing.(although I dont know is this approach is Best Way or not - but this is not my Question!)

public static void RegisterRoutes(RouteCollection routes)
        {
            routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

            routes.MapRoute(
                name: "interface",
                url: "soal/{id}/{slug}",    /* "soal" is only decor */
                defaults:new { controller = "ui", action = "singleQuestion",
                    slug = UrlParameter.Optional}    /*I made "slug" optional because I don't need this part to retrieve conternt from database */
                                      /* slug only explains content of the webpage*/

                );

            routes.MapRoute(
                name: "Default",
                url: "{controller}/{action}/{id}",
                defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
            );
        }

This Is The "SingleQuestion" Action that Processes pages:

    public ActionResult singleQuestion(int id, string slug)
                                               /* "slug" parameter here does nothing. Action Does not use this.*/
{
    var questions = BQcnt.Questions;
    var showQuestion = questions.Find(id);
    if (showQuestion != null)
    {
        var AnswerOfShowQuestion = BQcnt.Answers.Where(
            i => i.QuestionID == showQuestion.QuestionID);

        ViewBag.lastQuestion = showQuestion;
        ViewBag.answer_of_show_question = AnswerOfShowQuestion;
    }
    return View(questions.OrderByDescending(x => x.QuestionID));
}

well this works fine when i use these Urls:

http://localhost:9408/soal/13/bla-bla-bla

and

http://localhost:9408/soal/13

these urls direct to same pages But My only problem is this: when I use second url, I want when page loads automatically URL changes to Complete One and the slug append to the url, Like first One.

I want something exactly Like stackoverflow.com urls. these two urls:

http://stackoverflow.com/questions/34621154/centering-multiple-textviews-programmatically-on-android

http://stackoverflow.com/questions/34621154

opens same page, But when we use second one(as you can examine ) url translates to first one automatically. but my urls does not translate like this.