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XPath and XML Namespaces

It's great that in C# code you can easily reference XML nodes using the XmlDocument's built-in XPath support:

XmlNode titleNode = doc.SelectSingleNode("/html/head/title");

But have you ever run into the situation where you call SelectSingleNode or SelectNodes and you get back nothing when you can see there are nodes there? The problem might be that the document is using XML Namespaces.

Consider this XML snippet:

<urlset xmlns="">

If you attempt to retrieve nodes from this document using this code:

XmlNode urlNodes = doc.SelectNodes("/urlset/url");

You might expect to get back a set of all the url nodes, but instead you get back nothing.

To query into a document that uses namespaces, you need to use an XmlNamespaceManager. The namespace manager takes care of mapping a prefix to a namespace, and lets you associate the nodes in the query with particular namespaces.

The following code would query the sitemap.xml:

XmlNamespaceManager nsMgr = new XmlNamespaceManager(myXmlDoc.NameTable);
nsMgr.AddNamespace("sm", "");
XmlNode urlNodes = doc.SelectNodes("/sm:urlset/sm:url");

If a page uses multiple namespaces then you'd have multiple calls to AddNamespace with different prefixes. Your query string can include nodes from different namespaces.

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