How to retrieve data from XML file after Schema validation?

Question

In my XML file, it has this line with the top element. xmlns="http://tempuri.org/XMLSchema.xsd". I think this line it is used to conform with schema, Because I specified ValidationType in XmlValidatingReader instance to ValidationType.Schema. However, if I use xpath (XmlDocument's SelectSingleNode method ) to select node(s). It would not find any thing at all. Once I delete "xmlns="http://tempuri.org/XMLSchema.xsd"" from the XML file, it works fine? Could anyone please point out why I cannot retrieve data by Xpath after schema validation? Thank you very much