public static string UrlEncode(this string s)
        {
            StringBuilder result = new StringBuilder();
            foreach (char c in s)
            {
                if (char.IsLetterOrDigit(c))
                {
                    result.Append(c);
                }
                else
                {
                    result.Append("%" + ((int)c).ToString("X2"));
                }
            }
            return result.ToString();
        }