protected void b_decode_Click(object sender, EventArgs e)
    {
        string base64text = tb_base64.Text;
        if (IsBase64String(base64text))
        {
            string urlDecoded = (Encoding.UTF8.GetString(Convert.FromBase64String(base64text)));
            l_result.Text = urlDecoded;
       }
    }
    private static bool IsBase64String(string s)
    {
        s = s.Trim();
        if (!Regex.IsMatch(s, @"^[a-zA-Z0-9\+=/]*$", RegexOptions.None)) return false;
        if (s.Length % 4 != 0)
        {
            return false;
        }
        return true;
    }