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

Chapter 2: XML ReadersIn the Microsoft .NET Framework, two distinct sets of classes provide for XML-drivenreading and writing operations. These classes are known globally as XML readers andwriters. The base class for readers is XmlReader, whereas XmlWriter provides the baseprogramming interface for writers. In this chapter, we'll focus on a particular type ofXML readers—the XML text readers. In Chapter 3, we'll zero in on validating readersand then move on to XML writers in Chapter 4.The Programming Interface of ReadersXmlReader is an abstract class available from the System.Xml namespace. It definesthe set of functionalities that an XML reader exposes to let developers access an XMLstream in a noncached, forward-only, read-only way.An XML reader works on a read-only stream by jumping from one node to the next in aforward-only direction. The XML reader maintains an internal pointer to the currentnode and its attributes and text but has no notion of previous and next nodes. You can'tmodify text or attributes, and you can move only forward from the current node. If youare visiting attribute nodes, however, you can move back to the parent node or accessan attribute by index. The visit takes place in node-first order, but other visitingalgorithms can be arranged in custom reader classes. See the note on page 72 formore information about visiting algorithms.The specification for the XmlReader class recommends that any derived class shouldcheck at least whether the XML source is well-formed and throw exceptions if an erroris encountered. XML exceptions are handled through the tailor-made XmlExceptionclass. The XMLReader class specification does not say anything about XML validation.Throughout this chapter, you'll see that the .NET Framework provides several readerclasses with and without validation capabilities. Valid sources for an XML reader aredisk files as well as any flavor of .NET streams and text readers (for example, stringreaders).An OOP RefresherThroughout this book, I'll often use terms such as interface and class, sometimesqualified by helper adjectives such as abstract or base. Although a full explanation ofthese terms and their related object-oriented programming (OOP) concepts is beyondthe scope of this book, a quick terminology refresher will help you get to the heart ofthe XML class hierarchy in the .NET Framework.In the .NET Framework, an interface is a container for a named collection of method,property, and event definitions referred to as a contract An interface can be used as areference type, but it is not a creatable type. Other types can implement one or moreinterfaces. In doing so, they adhere to the interface's contract and agree to provideactual implementation for all the methods, properties, and events in the contract.A class is a container that can include data and function members (methods,properties, events, operators, and constructors). Classes support inheritance fromother classes as well as from interfaces. Any class from which another class inherits iscalled a base class.An abstract class simply declares its members without providing any implementation.Like interfaces, abstract classes are not creatable but can be used as reference types.An abstract class differs from an interface in that it has a slightly richer set of internalmembers (constructors, constants, and operators). Members of an abstract class canbe scoped as private, public, or protected, whereas members of an interface aremostly public. In addition, child classes can implement multiple interfaces but caninherit from only one class.21