c# with XmlDocument and xpaths

Im almost embarrassed to admit it but I have spend almost a day playing around with the XmlDocument .NET class and xpaths. My problem was that I have the XML shown below(only a snipped of it is pasted below)

And wanted to get hold of the "id" tag inside patient. To do this I found that the fast way was to use the method SelectSingleNode to and a xpath expression like the following:

"/ClinicalDocument/recordTarget/patientRole/id".

In the following way:

XmlDocument xml = new XmlDocument();
xml.LoadXml(cda.Xmlcda);
XmlNode node = xml.SelectSingleNode("/ClinicalDocument/recordTarget/patientRole/id");

But no element was found. I tried quite a lot of different stuff. But nothing worked.

Until finally I found this article: (link) (which I only skimmed(to be completely honest))which described the funny thing that even if the elements is not prefixed in the XML the base namespace is used anyway. So to enable me to Xpath my way to the patient Id tag I needed a XmlNameSpaceManager to handle the namespaces.


The working code looks like this:
XmlDocument xml = new XmlDocument();
xml.LoadXml(cda.Xmlcda);
XmlNamespaceManager nsmgr = new XmlNamespaceManager(xml.NameTable);
nsmgr.AddNamespace("cda", "urn:hl7-org:v3");
XmlNode node = xml.SelectSingleNode("/ClinicalDocument/recordTarget/patientRole/id", nsmgr);



The XML:

<!-- ?xml-stylesheet type="text/xsl" href="CDA.xsl"? -->
<!-- Readers should be aware of the evolving "Using SNOMED CT in HL7 Version 3" implementation guide, currently in a draft state. The guide, co-developed by HL7 and the College of American Pathologists, will be balloted by HL7 as an Informative Document. Recommendations in the final published guide should usurp patterns of SNOMED CT usage found in this sample instance. -->
<clinicaldocument xmlns="urn:hl7-org:v3" voc="urn:hl7-org:v3/voc"
xsi="http://www.w3.org/2001/XMLSchema-instance"
schemalocation="urn:hl7-org:v3 CDA.xsd">
<!-- ******************************************************** CDA Header ******************************************************** -->
<typeid root="2.16.840.1.113883.1.3" extension="POCD_HD000040">
<templateid root="2.16.840.1.113883.3.27.1776">
<id extension="c266" root="2.16.840.1.113883.19.4">
<code code="11488-4" codesystem="2.16.840.1.113883.6.1" codesystemname="LOINC" displayname="Consultation note">
<title>Good Health Clinic Consultation Note</title>
<effectivetime value="20000407">
<confidentialitycode code="N" codesystem="2.16.840.1.113883.5.25">
<languagecode code="en-US">
<setid extension="BB35" root="2.16.840.1.113883.19.7">
<versionnumber value="2">
<recordtarget>
<patientrole>
<id extension="12345" root="2.16.840.1.113883.19.5">
<patient>
<name>
</name></patient></id></patientrole></recordtarget></versionnumber></setid></languagecode></confidentialitycode></effectivetime></code>