How do I write a route that allows for underscores in ASP.NET MVC?

I received the following problem on the MCTS exam. The book says the answer is C, but I thought B would be the correct answer. Why is it C?

You create a new ASP.NET MVC 2 Web application. The following default routes are created in the Global.asax.cs file. (Line numbers are included for reference only.)

01 public static void RegisterRoutes(RouteCollection routes)
02 {
03 routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
04
05 routes.MapRoute( "Default", "{controller}/{action}/{id}", new { controller = "Home", action = "Index", id = "" } );
06 }

You implement a controller named HomeController that includes methods with the following signatures.

public ActionResult Index() 
public ActionResult Details(int id) 
public ActionResult DetailsByUsername(string username) 

You need to add a route to meet the following requirements:

• The details for a user must to be displayed when a user name is entered as the path by invoking the DetailsByUsername action.

• User names can contain alphanumeric characters and underscores, and can be between 3 and 20 characters long. What should you do?

A. Replace line 05 with the following code segment.

routes.MapRoute( "Default", "{controller}/{action}/{id}", new { controller = "Home", action = "DetailsByUsername", id = "" } );

B. Replace line 05 with the following code segment.

routes.MapRoute( "Default", "{controller}/{action}/{username}", new { controller = "Home", action = "DetailsByUsername", usern开发者_JS百科ame = "" }, new { username = @"\w{3,20}" } );

C. At line 04, add the following code segment.

routes.MapRoute( "Details by Username", "{username}", new { controller = "Home", action = "DetailsByUsername" }, new { username = @"\w{3,20}" } );

D. At line 04, add the following code segment.

routes.MapRoute( "Details by Username", "{id}", new { controller = "Home", action = "DetailsByUsername" }, new { id = @"\w{3,20}" } );

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When an ASP.NET MVC route looks for an action to map to, it takes several things into account:

1. The action name (Non-case sensitive)
2. The parameter names (non-case sensitive)
3. Whether or not the constraints are matched on the route.

Because of that, if you want a route that matches the criteria you listed, this route would be the correct answer:

routes.MapRoute( 
    "Details by Username", 
    "{username}", 
    new { controller = "Home", action = "DetailsByUsername" }, 
    new { username = @"\w{3,20}" } 
);

A URL with this structure would match that route for the following reasons:

http://example.com/my_user_name

1. The action name matches,
2. It's looking for a parameter in the action called 'username', and
3. The criteria for non-alphanumeric characters is met.

C is the correct answer as it adds to the current routes.

Answer B replaces the existing route definition, thus preventing the default routing to the Home controller Index action.

---

C is right.

For B, if you replace line 05, then all your other routes won't work. You also need to insert at line 04 because if not, the line 05 route will catch it before it even gets a chance.

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Replacing the route would make Details not work, since Id is not mapped any more.

C is the correct one