absolute.cs

//Your ValuesController.cs

//Version 1
string uri = Url.Route("Default", new { controller="Home", action="MyAction", id = 1 });
string absoluteUrl = new Uri(Request.RequestUri, uri).AbsoluteUri;
//http://localhost/Home/MyAction/1

//Version 2
string uri = Url.Route("DefaultApi", new { id = 1 });
string absoluteUrl = new Uri(Request.RequestUri, uri).AbsoluteUri;
//http://localhost/api/Values/1

//Your Global.asax

routes.MapHttpRoute(
    name: "DefaultApi",
    routeTemplate: "api/{controller}/{id}/{action}",
    defaults: new {action = "Index", id = RouteParameter.Optional }
);

routes.MapRoute(
    name: "Default",
    url: "{controller}/{action}/{id}",
    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);