What is a good way to find a specific value in an XML document using C#?

<env:Envelope
  xmlns:env="http://schemas.xmlsoap.org/soap/envelope/"
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:ns0="http://dev1/MyWebService1.wsdl">
 <env:Header>
  <wsse:Security
    xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd"
    xmlns="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd"
    xmlns:env="http://schemas.xmlsoap.org/soap/envelope/"
    env:mustUnderstand="1"/>
 </env:Header>
 <env:Body>
  <ns0:getItemNbrByItemIdResponseElement>
   <ns0:result>1010603</ns0:result>
  </ns0:getItemNbrByItemIdResponseElement>
 </env:Body>
</env:Envelope>

XNamespace ns0 = "http://dev1/MyWebService1.wsdl";

String result = doc.Descendants(ns0 + "result").First().Value;

XPathDocument xmldoc = new XPathDocument(@"C:tmpsample.xml");
XPathNavigator nav = xmldoc.CreateNavigator();

XmlNamespaceManager nsMgr = new XmlNamespaceManager(nav.NameTable);
nsMgr.AddNamespace("ns0", "http://dev1/MyWebService1.wsdl");

XPathNavigator result = nav.SelectSingleNode("//ns0:result", nsMgr);
System.Diagnostics.Debug.WriteLine(result.Value);

XmlDocument doc = new XmlDocument();
doc.PreserveWhitespace = true;

XmlNamespaceManager mgr = GetNamespace(doc);
doc.LoadXml(xmltext);

XmlNode nd = doc.DocumentElement.SelectSingleNode("//ns0:result", mgr);

private XmlNamespaceManager GetNamespace(XmlDocument document)
{
    XmlNamespaceManager mgr = new XmlNamespaceManager(document.NameTable);
    mgr.AddNamespace("ns0", "http://dev1/MyWebService1.wsdl");
    return mgr;
}

XmlDocument xmlDoc = new XmlDocument();

XNamespace ns0 = "http://dev1/MyWebService1.wsdl";

xmlDoc.Load(request.GetResponse().GetResponseStream());

XDocument xDoc = XDocument.Load(new XmlNodeReader(xmlDoc));

String result = xDoc.Descendants(ns0 + "result").First().Value;

XPathNavigator navigator = document.CreateNavigator();

        string res = (string)navigator.Evaluate("normalize-space(/)");