Namesapce with xpath query

        public XmlNode Query(string queryRestriction)
        {
            XmlNamespaceManager nameSpaceManager = new XmlNamespaceManager(this._doc.NameTable);
            nameSpaceManager.AddNamespace("w", XMLFILE_NAMESPACE);
            XmlNode customerElement = this._doc.SelectSingleNode("/w:payments/w:customer[@" + CUSTOMER_CD + "='" + queryRestriction + "']",
                nameSpaceManager);
            return customerElement;
        }