XML Namespace, can not select nodes

I have this xml:

<?xml version="1.0" encoding="utf-8" ?>  
    <ArrayOfFileInformation xmlns:xsd="http://www.w3.org/2001/XMLSchema"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns="http://tempuri.org/">  <Data> ......

When I leave "xmlns="http://tempuri.org/" in the Xml, the following doesn't return any nodes:

CString tag = L"//Data";
MSXML2::IXMLDOMNodeListPtr pDataTag = pXMLDocument->selectNodes((_bstr_t)tag);

When I remove the attribute, everything works fine though.

Can you please help me?

Solution

In order to select namespaced nodes in XPath you need to use the facility provided by your XPath library to bind a prefix to the relevant namespace URI, and then use that prefix when selecting nodes by name. The MSDN documentation for the selectNodes method has a C++ example of just this, the key is to set the SelectionNamespaces property on the document

pXMLDocument->setProperty("SelectionNamespaces", "xmlns:tmp='http://tempuri.org/'");

which will then allow you to use an XPath of //tmp:Data