class MySignedXml : SignedXml
    {
        public MySignedXml(XmlDocument document)
            :base(document)
        {
        }

        public override XmlElement GetIdElement(XmlDocument document, string idValue)
        {
            XmlNameTable myXmlNameTable = new NameTable();
            XmlNamespaceManager myNamespacemanager = new XmlNamespaceManager(myXmlNameTable);
            myNamespacemanager.AddNamespace( "wsu", 
                    "http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" );
            XmlNodeList lst = document.SelectNodes("//*[@wsu:Id='" + idValue + "' or @wsu:ID='" + idValue + 
                    "' or @wsu:ID='" + idValue + "']", myNamespacemanager);
            if (lst.Count != 1)
                return null;
            return (XmlElement)lst.Item(0);
        }
    }