ASP.Net MVC uses the new (to ASP.Net 3.5) Http*Base wrapper classes (HttpContextBase, HttpRequestBase, HttpResponseBase, etc) instead of the original Http* classes.  This allows you to create mock implementations that inherit the Http*Base classes without an actual HTTP request.  This is useful for unit testing, and for overriding standard behaviors (such as route checking).

In ordinary MVC code, the HttpContext, Request, and Response properties will return Http*Wrapper instances that directly wrap the original Http* classes (eg, HttpContextWrapper, which wraps HttpContext).  Most MVC developers use the HttpContext and related properties without being aware of any of this redirection.

Until you call Response.RedirectToRoute.  This method, which is new to .Net 4.0, redirects the browser to a URL for a route in the new ASP.Net routing engine.  Like other HttpResponse methods, HttpResponseBase has its own version of this method for derived classes to override.

However, in .Net 4.0, Microsoft forgot to override this method in the standard HttpResponseWrapper.  Therefore, if you call Response.RedirectToRoute in an MVC application (where Response is actually an HttpResponseWrapper), you’ll get a NotImplementedException.

You can see this oversight in the methods list for HttpResponseWrapper.  Every method except for RedirectToRoute and RedirectToRoutePermanent are list as (Overrides HttpResponseBase.MethodName().); these methods are listed as (Inherited from HttpResponseBase.MethodName().)

To work around this issue, you can either use the original HttpResponse by writing HttpContext.Current.Response.RedirectToRoute(…) or by calling Response.Redirect instead.

Note that most MVC applications should not call Response.Redirect or Response.RedirectToRoute at all; instead, they should return ActionResults by calling helper methods like return Redirect(…); or return RedirectToAction(…);

In the upcoming ASP.Net 4.5 release, these methods have been properly overridden.