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

John Pollard — Sat, 12 Nov 2005 17:26:00 GMT

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!

John Pollard's Technical Blog : XML

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