SelectSingleNode returning null for known good xml node path using XPath

Consider this simple XML document. The serialized XML shown here is the result of an XmlSerializer from a complex POCO object whose schema I have no control over.

<My_RootNode xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns="">
  <id root="2.16.840.1.113883.3.51.1.1.1" extension="someIdentifier" xmlns="urn:hl7-org:v3" /> 
  <creationTime xsi:nil="true" xmlns="urn:hl7-org:v3" />      
</My_RootNode>

The goal is to extract the value of the extension attribute on the id node. In this case, we are using the SelectSingleNode method, and given an XPath expression as such:

XmlNode idNode = myXmlDoc.SelectSingleNode("/My_RootNode/id");
//idNode is evaluated to null at this point in the debugger!
string msgID = idNode.Attributes.GetNamedItem("extension").Value;

The problem is that the SelectSingleNode method returns null for the given XPath expression.

Question: any ideas on this XPath query's correctness, or why this method call + XPath expression would return a null value? Perhaps the namespaces are part of the problem?

I strongly suspect the problem is to do with namespaces. Try getting rid of the namespace and you'll be fine - but obviously that won't help in your real case, where I'd assume the document is fixed.

I can't remember offhand how to specify a namespace in an XPath expression, but I'm sure that's the problem.

EDIT: Okay, I've remembered how to do it now. It's not terribly pleasant though - you need to create an XmlNamespaceManager for it. Here's some sample code that works with your sample document:

using System;
using System.Xml;

public class Test
{
    static void Main()
    {
        XmlDocument doc = new XmlDocument();
        XmlNamespaceManager namespaces = new XmlNamespaceManager(doc.NameTable);
        namespaces.AddNamespace("ns", "urn:hl7-org:v3");
        doc.Load("test.xml");
        XmlNode idNode = doc.SelectSingleNode("/My_RootNode/ns:id", namespaces);
        string msgID = idNode.Attributes["extension"].Value;
        Console.WriteLine(msgID);
    }
}

If you want to ignore namespaces completely, you can use this:

static void Main(string[] args)
{
    string xml =
        "<My_RootNode xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns=\"\">\n" +
        "    <id root=\"2.16.840.1.113883.3.51.1.1.1\" extension=\"someIdentifier\" xmlns=\"urn:hl7-org:v3\" />\n" +
        "    <creationTime xsi:nil=\"true\" xmlns=\"urn:hl7-org:v3\" />\n" +
        "</My_RootNode>";

    XmlDocument doc = new XmlDocument();
    doc.LoadXml(xml);

    XmlNode idNode = doc.SelectSingleNode("/*[local-name()='My_RootNode']/*[local-name()='id']");
}