Well I look in the documentation and HttpContext don't seem to have a Page member, so..
We'll I have to see if it compiles.

True to tell there is a CurrentHandler property though, which I could cast to a page.
But... although it's likely there would be a current page, it's not alway true.

Anyway I end up doing some code like that:

public class WebUrl
{
   public static string GetRoot()
   {
       HttpContext context = HttpContext.Current;

       string port = context.Request.ServerVariables["SERVER_PORT"];
       if (port == null || port == "80" || port == "443")
           port = "";
       else
           port = ":" + port;

       string protocol = context.Request.ServerVariables["SERVER_PORT_SECURE"];
       if (protocol == null || protocol == "0")
           protocol = "http://";
       else
           protocol = "https://";

       string ret = string.Format("{0}{1}{2}{3}",
           protocol,
       context.Request.ServerVariables["SERVER_NAME"],
           port,
           context.Request.ApplicationPath);
       if (!ret.EndsWith("/"))
           ret = ret + "/";

       return ret;
   }
   public static string GetUrl(string file)
   {
       if (file.StartsWith("~/"))
           file = file.Substring(2);
       else if (file.StartsWith("/"))
           file = file.Substring(1);
   
       return HttpUtility.UrlPathEncode(GetRoot() + file);
   }
}

Nice function.
And shorter that the one I wrote.
Thanks ;-)