A string that represents the namespace URI.
The namespace URI returned depends on the System.Xml.XPath.XPathNodeType of the current node.
The XPathNavigator.NamespaceURI property returns urn:samples for the node <bk:book xmlns:bk='urn:samples'>.
Only XPathNodeType.Element and XPathNodeType.Attribute System.Xml.XPath.XPathNodeType nodes can have a namespace URI.