Working with XDocument and Namespaces

I really like working with the new LINQ support for XML files. However, every time I work with a document that has namespaces, I stumble. For some reason, I just can’t remember all the details required to be able to use XPath expressions when you have namespaces defined in an XML file. A blog post, therefore, seems like a good idea.

Default Namespace

A common case is where an XML file has a default namespace defined at the very top. For example,

<Project ToolsVersion="3.5" DefaultTargets="Build" 
    xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <RootNamespace>Test</RootNamespace>
    <AssemblyName>Test</AssemblyName>
    <OutputType>Library</OutputType>
  </PropertyGroup>
  ...

I wanted to retrieve the RootNamespace element, which without the default namespace can be done with code like this:

return doc.Root.XPathSelectElement("PropertyGroup/RootNamespace").Value

But when you have a default namespace, you’ll get an exception because the XPathSelectElement method returns null. Instead, you have to use a namespace manager like this:

XmlNamespaceManager namespaces = new XmlNamespaceManager(new NameTable());
XNamespace ns = _doc.Root.GetDefaultNamespace();
namespaces.AddNamespace("ns", ns.NamespaceName);
return doc.Root.XPathSelectElement("ns:PropertyGroup/ns:RootNamespace", namespaces).Value; 

The trick, as you can see, is to give a name (“ns” in this example) to the default namespace, and then explicitly refer to that name in your XPath expression.