static string fname = string.empty

 private static void xmlparser6(string xmldata)
        {
            XDocument xml = XDocument.Parse(xmldata);

            var q = from b in xml.Descendants("client")
                    select new
                    {
                        fname = b.Element("firstname").Value,
                        lname = b.Element("lastname").Value,
                        email = b.Element("email").Value,
                    };

            if (fname != "")
            {
                MessageBox.Show(fname);
            }

        }

// XML
<?xml version="1.0" encoding="iso-8859-7"?>
<whmcsapi version="5.1.2">
<client>
  <userid>1</userid>
  <id>1</id>
  <firstname>john</firstname>
  <lastname>deer</lastname>
  <companyname>that one company</companyname>
  <email>noemail@email.com</email>
</client>
</whmcsapi>