Applied XML Programming for Microsoft .NET.pdf - Csbdu.in

methods are not part of the official W3C XML DOM specification but represent, instead,Microsoft extensions to the standard XML DOM.At the application level, XML DOM methods and the XPath navigator supply differentprogramming interfaces, but internally they run absolutely equivalent code.The SelectNodes Internal ImplementationThe SelectNodes method internally employs a navigator object to retrieve the list ofmatching nodes. The return value of the navigator's Select method is then used toinitialize an undocumented internal node list class named System.Xml.XPath.XPathNodeList. As you have probably guessed, this class inherits from XmlNodeList,which is a documented class. To verify this statement, compile and run the followingsimple code:XmlDocument doc = new XmlDocument();doc.Load(fileName);XmlNodeList nodes = doc.SelectNodes("child::*");Console.WriteLine(nodes.ToString());The true type of the variable nodes is XPathNodeList. If you try to reference that type inyour code, you get a compile error due to the protection level of the class.What's the difference between using SelectNodes and the XPath navigator object? TheSelectNodes method uses a navigator that works on top of a generic XML documentclass—the XmlDocument class. The SelectNodes method's navigator object is, in fact,created by the XmlDocument class's CreateNavigator method. If you choose to publiclymanage a navigator, you normally create it from a more specific and XPath-optimizeddocument class—the XPathDocument class.The XPath expression is passed to the navigator as plain text:XmlNodeList SelectNodes(string xpathExpr, XmlNamespaceManagernsm)Interestingly enough, however, if you use this overload of the SelectNodes method thathandles namespace information, the XPath expression is first compiled and thenpassed to the processor.As we'll see in the section "Compiling Expressions," on page 274, only compiled XPathexpressions support namespace information. In particular, they get namespaceinformation through an instance of the XmlNamespaceManager class.The SelectSingleNode Internal ImplementationThe SelectSingleNode method is really a special case of SelectNodes. Unfortunately,there is no performance advantage in using SelectSingleNode in lieu of SelectNodes.The following pseudocode illustrates the current implementation of theSelectSingleNode method:public XmlNode SelectSingleNode(string xpathExpr){XmlNodeList nodes = SelectNodes(xpathExpr);return nodes[0];}The SelectSingleNode method internally calls SelectNodes and retrieves all the nodesthat match a given XPath expression. Next it simply returns the first selected node to209