Converts a virtual path to an application absolute path - Extension Methods

Converts the provided app-relative path into an absolute Url containing the full host name

   /// App-Relative path
    /// Provided relativeUrl parameter as fully qualified Url
    /// ~/path/to/foo to http://www.web.com/path/to/foo
    public static string GetAbsoluteUrl(string relativeUrl)
    {
        //VALIDATE INPUT
        if (String.IsNullOrEmpty(relativeUrl))
            return String.Empty;

        //VALIDATE INPUT FOR ALREADY ABSOLUTE URL
        if (relativeUrl.StartsWith("http://", StringComparison.OrdinalIgnoreCase) || relativeUrl.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
            return relativeUrl;

        //VALIDATE CONTEXT
        if (HttpContext.Current == null)
            return relativeUrl;

        //GET CONTEXT OF CURRENT USER
        HttpContext context = HttpContext.Current;

        //FIX ROOT PATH TO APP ROOT PATH
        if (relativeUrl.StartsWith("/"))
        {
            relativeUrl = relativeUrl.Insert(0, "~");
        }

        //GET RELATIVE PATH
        Page page = context.Handler as Page;
        if (page != null)
        {
            //USE PAGE IN CASE RELATIVE TO USER'S CURRENT LOCATION IS NEEDED
            relativeUrl = page.ResolveUrl(relativeUrl);
        }
        else //OTHERWISE ASSUME WE WANT ROOT PATH
        {
            //PREPARE TO USE IN VIRTUAL PATH UTILITY
            if (!relativeUrl.StartsWith("~/"))
            {
                relativeUrl = relativeUrl.Insert(0, "~/");
            }

            relativeUrl = VirtualPathUtility.ToAbsolute(relativeUrl);
        }

        var url = context.Request.Url;
        var port = url.Port != 80 ? (":" + url.Port) : String.Empty;

        //BUILD AND RETURN ABSOLUTE URL
        return String.Format("{0}://{1}{2}{3}",
               url.Scheme, url.Host, port, relativeUrl);
    }