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

• Connection model Both readers and navigators work on top of a datasource. Readers, however, work connected to the input stream, which isoften a persistent storage medium like a disk file. Navigators always workon memory-mapped data sources like XML DOM or more optimized andspecialized structures. Readers must be closed when you have finishedwith them; navigators are simply garbage-collected when they go out ofscope. A parallel can be drawn with ADO.NET data readers and DataSetobjects. An XML data reader object, like the SqlDataReader base class, isconnected to the data source, whereas a DataSet object is a disconnectedobject.• Navigation interface Readers are simple read-only and forward-onlycursors. Navigators too are read-only, but they let you move forward andbackward. The navigator's set of move methods is significantly richer. Inparticular, the set includes methods for going to the root of the underlyingdocument, for reaching the parent node, for reaching the next and theprevious sibling, for reaching the node where the given namespace isdefined, and even more. In addition, you can synchronize the navigatorposition with the current position on another navigator object.• Programming interface Navigators provide rich XPath capabilities andsupply methods that perform XPath queries and return groups of relatednodes. You have a generic Select method but also ad hoc selectionmethods that specialize on the most common XPath axes, such asdescendant, ancestor, and child. In addition, navigators can simplyevaluate an XPath expression and return the value.Conceptually, XPath navigators and XML readers occupy diametrically opposedpositions in the .NET XML puzzle. Moreover, this difference clearly stems from theirnames. Navigators are thought to traverse XML-based or XMLlooking data. XMLreaders are simply lower-level tools that you can use to read XML-based or XMLlookingdata and build in-memory data structures that navigators rely on.NoteAs mentioned, XML readers and navigators work on XMLbased orXML-looking data. XML-based data refers to data persisted, or justread, as well-formed XML. As we saw in Chapter 2, however, youcan use specialized reader classes to publish non-XML datathrough a virtual XML tree. Likewise, a navigator can be built towork on top of a data store that creates a virtual XML tree from non-XML data. XML-looking data refers to just such virtual XML trees.The XPathNavigator Programming InterfaceLet's briefly review the properties and methods that form the programming interface ofthe XPathNavigator class. A valid instance of the class can be obtained by calling theCreateNavigator method on any .NET Framework class that implements theIXPathNavigable interface.Properties of the XPathNavigator ClassTable 6-3 summarizes the properties of the XPathNavigator class. As you can see,most of these properties reflect the characteristics of the currently selected node.Table 6-3: Properties of the XPathNavigator ClassPropertyDescriptionBaseURIGets the base URI of the current node219