Xml-SelectNodes with default-namespace via XmlNamespaceManager not working as expected

by 
// This will fail with dotnet 3.5sp1. Why? 
//!!!! 
Assert.AreEqual(2, doc.SelectNodes("//b", nsmgr).Count);

This is a FAQ. In XPath any unprefixed name is assumed to be in “no namespace”. In order to select elements that belong to a namespace, in any XPath expression their names must be prefixed with a prefix that is associated with this namespace. The AddNamespace() method serves exactly this purpose. It creates a binding between a specific namespace and a specific prefix. Then, if this prefix is used in an XPath expression, the element prefixed by it can be selected.

It is written in the XPath W3C spec: “A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. This is the same way expansion is done for element type names in start and end-tags except that the default namespace declared with xmlns is not used: if the QName does not have a prefix, then the namespace URI is null”.

See this at: w3.org/TR/xpath/#node-tests .

So, any unprefixed name is considered to be in “no namespace”. In the provided XML document there are no b elements in “no namespace” and this is why the XPath expression //b selects no nodes at all.

Use:

XmlNamespaceManager nsmanager = new XmlNamespaceManager(doc.NameTable);
nsmanager.AddNamespace("x", "urn:test.Schema");

and later:

Assert.AreEqual(2, doc.SelectNodes("//x:b", nsmanager).Count);

Remember: The whole purpose of registering the namespace is to be able to use the prefix (in this case x) in any XPath expression.

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