I upgraded my project to ASP.NET 4 RTM with ASP.NET MVC 2.0 RTM today.

I was previously using ASP.NET 3.5 with ASP.NET MVC 2.0 RTM.

Some of my routes don't work suddenly and I don't know why. I'm not sure if something changed between 3.5 and 4.0 - or if this was a regression type issue in the 4.0 RTM. (I never previously tested my app with 4.0).

I like to use Url.RouteUrl("route-name", routeParams) to avoid ambiguity when generating URLs. Here's my route definition for a gallery page. I want imageID to be optional (you get a thumbnail page if you don't specify it).

// gallery id
routes.MapRoute(
    "gallery-route",
    "gallery/{galleryID}/{imageID}/{title}",
    new { controller = "Gallery", action = "Index", 
          galleryID = (string) null, 
          imageID = (string) null, 
          title = (string) null}
);

In .NET 3.5 / ASP.NET 2.0 RTM / IIS7

Url.RouteUrl("gallery-route", "cats")                =>  /gallery/cats
Url.RouteUrl("gallery-route", "cats", 4)             =>  /gallery/cats/4
Url.RouteUrl("gallery-route", "cats", 4, "tiddles")  =>  /gallery/cats/4/tiddles

In .NET 4.0 RTM / ASP.NET 2.0 RTM / IIS7

Url.RouteUrl("gallery-route", "cats")                =>  null
Url.RouteUrl("gallery-route", "cats", 4)             =>  /gallery/cats/4
Url.RouteUrl("gallery-route", "cats", 4, "tiddles")  =>  /gallery/cats/4/tiddles

Previously I could supply only the galleryID and everything else would be ignored in the generated URL. But now it's looking like I need to specify all the parameters up until title - or it gives up in determining the URL.

Incoming URLs work fine for /gallery/cats and that is correctly mapped through this rule with imageID and title both being assigned null in my controller.

I also tested the INCOMING routes with http://haacked.com/archive/2008/03/13/url-routing-debugger.aspx and they all work fine.