ASP.NET MVC Url.Action adds current route values to generated url

Question

I have seen this question a couple of times here in SO but none of them with any acceptable answer:

ASP.NET MVC @Url.Action includes current route data
ASP.NET MVC implicitly adds route values

Basically I have Controller with an action method called Group, it has an overload that receives no parameters and displays a list of elements and another one that receives an id and displays details for that group.

If I do something like this:

Url.Action("Group", "Groups"); 

From the main page of the site (/) it returns an url like this:

"mysite.com/Groups/Group" 

which is alright Now, if the current address of the site is /Groups/Group/1 And I call the same method

Url.Action("Group", "Groups"); 

the returned url is this:

"mysite.com/Groups/Group/1" 

It automatically adds the value of the route for the current page when generating the URL. Even if I generate the URL this way:

Url.Action("Group", "Groups", null); 

Thus explicitly specifying that I don't want any route values, the generated URL is the same. To get the address I want I have to explicitly set the route value to an empty string, like so:

Url.Action("Group", "Groups", new {id=""}); 

This will generate the following url:

"mysite.com/Groups/Group" 

My question is, why does this happen? If I don't set any route values it shouldn't add them to the generated URL.

Answer 1

Url.Action will reuse the current request parameters, if you do not explicitly set them. It is by design in outbound url-matching algorithm. When looking for the route data parameters in a process of generating url, parameters are taken from:

1) explicitly provided values

2) values from the current request

3) defaults

In the order I specified above.

Outbound matching algorithm for routes is complicated, so it is good practice to explicitly set all parameters for request, as you did in your example

Answer 2

My application explicitly sets route values and does not want a magical value from the current request. I want to be in full control.

I have made an extension which coexists with my route library collection. Hence the single RouteValueDictionary param. (See my Route library comment at the bottom)

Here I remove any routevalues from the request prior to generating a url.

(note: for the array.contains ignorecase part, see: How can I make Array.Contains case-insensitive on a string array?)

public static string Action(this UrlHelper helper,                              RouteValueDictionary routeValues) {     RemoveRoutes(helper.RequestContext.RouteData.Values);      string url = helper.Action(routeValues["Action"].ToString(), routeValues);     return url; }  public static void RemoveRoutes(RouteValueDictionary currentRouteData) {     List<string> keyList = new List<string>(currentRouteData.Keys);      string[] ignore = new[] { "Area", "Controller", "Action" };     foreach (string key in keyList)     {         if (!ignore.Contains(key, StringComparer.CurrentCultureIgnoreCase))             currentRouteData.Remove(key);     } } 

I have Form and ActionLink extension methods that uses the RemoveRoutes method. No helper in my mvc library uses a method that is not an extension method i have created. Thereby, all routedata is cleaned up before generating urls.

For reference I use AttributeRouting. Here is an example of one route from my route library.

public static RouteValueDictionary DisplayNews(int newsId) {     RouteValueDictionary route = new RouteValueDictionary();     route["Area"] = _area;     route["Controller"] = _controller;     route["Action"] = "DisplayNews";     route["newsId"] = newsId;     return route; }