What is Ambient Route Values in ASP.NET MVC and how it works?

Question

I am new to ASP.NET MVC. I read Professional ASP.NET MVC 3 and it has two pages talk about Ambient Route Values but I don't understand how it works. I search "asp.net mvc ambient route values" on google and still not find any articles or websites that explain what is it or how it works.

I want to know what is "Ambient Route Values" in ASP.NET MVC? How it works?

Answer 1

Ambient route values are related to all those values that are not needed for the current route outbound processing.

Let's explain through an example

Take for instance this route definition:

routes.MapRoute(
    "Complex",
    "{securityArea}/{permission}/{action}/{id}",
    new { controller = "Administration", action = "List", id = UrlParameter.Optional }
);
routes.MapRoute(
    "Default",
    "{controller}/{action}/{id}",
    new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);

The scenario of ambient route values in this case would be:

1. User does some administration, so he's currently on URL served by the first route definition:

   /users/change/apply/45
   

2. He edits some form on this URL and posts data back.

3. When he hits the server (executes some controller action), all of those route values get populated and are part of the context's route values now.
4. Controller does what it has to do and in the end we want it to redirect to non-admin part of the application, so hitting the second Default route.

Now if we take a look of the URL generation in #4. What happens?

1. route values that are defined during request are:
   • controller = "Administration"
   • action = "Apply"
   • securityArea = "Users"
   • permission = "Change"
   • id = 45
2. Only the first two are needed to generate the URL of the second Default route
3. What happens with the rest ambient route values?

4. They get added to the URL as well:

   /Home/Index/?securityArea=Users&permission=Change
   

   And we don't want that.

That's why they're called ambient because they are just *hanging in there orphaned in the request. This is my explanation of ambient route values. Hopefully explained in an understandable way.

I've also written about removing these ambient values in one of my blog posts where I've provided a custom route class that does this removal.

As referred on page 232

Ambient route values in the book that you linked also refer to outbound route processing but it talks about ambient values as those that we don't need to supply for outbound route processing because they will be taken from current values (namely controller and action may as well be ambient values).

Book though doesn't talk about the problem with ambient route values that I outlined in my upper answer. All defined route values can be ambient and they may cause problems when we don't realise how routing works.