private static Uri ConvertToAbsoluteUri(string returnUrl, HttpContextBase context)
        {
            if (Uri.IsWellFormedUriString(returnUrl, UriKind.Absolute))
                return new Uri(returnUrl, UriKind.Absolute);

            if (!VirtualPathUtility.IsAbsolute(returnUrl))
                returnUrl = VirtualPathUtility.ToAbsolute(returnUrl);

            return new Uri(context.PublicUri(), returnUrl);
        }