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

How XPointer Uses XPathAn XPointer fragment identifier can be the name of a particular portion of the targetdocument, but it could also be a more complex and expressive XPath query. Forexample, you could link a piece of information using the following syntax:invoices.xml#xpointer(/descendant::invoice[@id=201])This expression references the particular descendant node named having anid attribute equal to 201.XPath in the XML DOMIn the .NET Framework, you can make use of XPath expressions in two ways: throughthe XML DOM or by means of a new and more flexible API based on the concept of theXPath navigator.In the former case, you use XPath expressions to select nodes within the context of aliving instance of the XmlDocument class. As we saw in Chapter 5, the XmlDocumentclass is the .NET Framework class that renders a given XML document as ahierarchical object model (XML DOM). This approach keeps the API close to the oldMSXML programming style and has probably been supplied mostly for compatibilityreasons.The alternative approach consists of creating an instance of the XPathDocument classand obtaining from it an XPath navigator object. The navigator object is a generic XPathprocessor that works on top of any XML data store that exposes the IXPathNavigableinterface. Rendered through the XPathNavigator class, the XPath navigator objectparses and executes expressions using its Select method. XPath expressions can bepassed as plain text or as preprocessed, compiled expressions. As you can see,although the classes involved are different, the overall programming style is not muchdifferent from those pushed by MSXML and the .NET Framework XML DOM classes.This said, though, the XPath navigator object represents a quantum leap from theSelectNodes method of the XmlDocument class. For one thing, it works on top of highlyspecialized document classes that implement IXPathNavigable and are optimized toperform both XPath queries and XSL transformations. In contrast, the XmlDocumentclass is a generic data container class that incorporates an XPath processor but is notbuilt around it.Several classes in the .NET Framework implement the IXPathNavigable interface, thusmaking their contents automatically selectable by XPath expressions. We'll look at thenavigation API in more detail in the section "The .NET XPath Navigation API," on page263. For now, let's review the XPath support built into the XmlDocument class.The XML DOM Node Retrieval APIWhen using XPath queries to query an XML DOM instance, you can use theSelectNodes method of the XmlDocument class. In particular, SelectNodes returns acollection that contains instances of all the XmlNode objects that match the specifiedexpression. If you don't need the entire node-set, but instead plan to use the query tolocate the root of a particular subtree, use the SelectSingleNode method.SelectSingleNode takes an XPath expression and returns a reference to the first matchfound.The SelectNodes and SelectSingleNode methods perform identical functionality to themethods available from the Component Object Model (COM)– based MSXML librarythat script and Microsoft Win32 applications normally use. It is worth noting that these208