private const string sampleDocument = @"<item xmlns='http://sample/'>
            <code>cod0123456789</code>
            <user>Taro</user>
            <user>Jiro</user>
            <user>Hanako</user>
            </item>";

        private void walk( XmlNode node )
        {
            if( node.NodeType == XmlNodeType.Element
                && node.LocalName == "user" && node.NamespaceURI == "http://sample/" )
            {
                System.Diagnostics.Trace.WriteLine(node.InnerText);
            }
            foreach( XmlNode child in node.ChildNodes )
            {
                walk( child );
            }
        }

        private void Form1_Load(object sender, System.EventArgs e)
        {
            XmlDocument doc = new XmlDocument();
            doc.LoadXml(sampleDocument);
            XmlNamespaceManager nsmgr  = new XmlNamespaceManager(doc.NameTable);
            nsmgr.AddNamespace("myns", "http://sample/");

            // 子を順番で決め打ち
            System.Diagnostics.Trace.WriteLine(doc.ChildNodes[0].ChildNodes[0].InnerText);
            // 木を歩く
            walk(doc);
            // GetElementsByTagNameを使う
            XmlNodeList list1 = doc.GetElementsByTagName("user","http://sample/");
            foreach( XmlNode node in list1 )
            {
                System.Diagnostics.Trace.WriteLine(node.InnerText);
            }
            // SelectNodesを使う
            XmlNodeList list2 = doc.SelectNodes("//myns:user",nsmgr);
            foreach( XmlNode node in list2 )
            {
                System.Diagnostics.Trace.WriteLine(node.InnerText);
            }
            // SelectSingleNodeを使う
            System.Diagnostics.Trace.WriteLine(doc.SelectSingleNode("//myns:code",nsmgr).InnerText);
        }