How do I use XPath with a default namespace with no prefix?

Question

What is the XPath (in C# API to XDocument.XPathSelectElements(xpath, nsman) if it matters) to query all MyNodes from this document?

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <MyNode xmlns="lcmp" attr="true">
    <subnode />
  </MyNode>
</configuration>

• I tried /configuration/MyNode which is wrong because it ignores the namespace.
• I tried /configuration/lcmp:MyNode which is wrong because lcmp is the URI, not the prefix.
• I tried /configuration/{lcmp}MyNode which failed because Additional information: '/configuration/{lcmp}MyNode' has an invalid token.

EDIT: I can't use mgr.AddNamespace("df", "lcmp"); as some of the answerers have suggested. That requires that the XML parsing program know all the namespaces I plan to use ahead of time. Since this is meant to be applicable to any source file, I don't know which namespaces to manually add prefixes for. It seems like {my uri} is the XPath syntax, but Microsoft didn't bother implementing that... true?

Answer 1

The configuration element is in the unnamed namespace, and the MyNode is bound to the lcmp namespace without a namespace prefix.

This XPATH statement will allow you to address the MyNode element without having declared the lcmp namespace or use a namespace prefix in your XPATH:

/configuration/*[namespace-uri()='lcmp' and local-name()='MyNode']

It matches any element that is a child of configuration and then uses a predicate filer with namespace-uri() and local-name() functions to restrict it to the MyNode element.

If you don't know which namespace-uri's will be used for the elements, then you can make the XPATH more generic and just match on the local-name():

/configuration/*[local-name()='MyNode']

However, you run the risk of matching different elements in different vocabularies(bound to different namespace-uri's) that happen to use the same name.