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Using SelectSingleNode (or SelectNodes) on XML where the default namespace has been set

Originally posted on my old MSDN blog

I've been stumped by this one at least two times over the last couple of years, so I thought it was a good candidate to be written up here.

I was trying to select a node from some standard XHTML where the default namespace was set. In otherwords the XHTML was something like:

!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"[]>
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>MSN Search News: Microsoft</title> ...

Note the xmlns attribute on the root <html> node.

Without thinking too hard, I first tried to find the title of the page by going ...

XmlDocument resultsXhtml = new XmlDocument();
resultsXhtml.Load("http://search.msn.com/news/results.aspx?q=Microsoft);
XmlNode metaNode = resultsXhtml.SelectSingleNode("//title");

... which left metaNode as null.

This took me a little while to figure out. Clearly I need to identify in the XPath query that the title tag is in the default namespace, but how can I do that if that namespace has no prefix in the actual XML.

The solution (reasonably obviously!) is to register a prefix of my own choosing in an XmlNamespaceManager object, and then use that namespace manager when doing the select. Here's some code that works:

XmlDocument resultsXhtml = new XmlDocument();
resultsXhtml.Load("http://search.msn.com/news/results.aspx?q=Microsoft");
XmlNamespaceManager namespaceManager = new XmlNamespaceManager(resultsXhtml.NameTable);
namespaceManager.AddNamespace("myprefix", "http://www.w3.org/1999/xhtml");
XmlNode metaNode = resultsXhtml.SelectSingleNode("//myprefix:title", namespaceManager);

I think what's interesting about this problem, is the way you have to think about namespaces and XPath queries. The namespace is a logical entity denoted by the URI not the prefix in the actual XML. Therefore you can register that URI with any prefix you want in your XPath, which isn't a completely intuitive concept - to me at least!

N.B. This blog post has now moved to https://writingontablets.com