1.4 Key Concepts

Before you can learn to work with XML in the .NET Framework, I have to introduce some of the key types you'll be using.

When using the DOM, as shown in Chapter 5, each node in an XML document is represented by an appropriately named class, starting with the abstract base class, XmlNode. Derived from XmlNode are XmlAttribute, XmlDocument, XmlDocumentFragment, XmlEntity, XmlLinkedNode, and XmlNotation. In turn, XmlLinkedNode has a number of subclasses that serve specific purposes (XmlCharacterData, XmlDeclaration, XmlDocumentType, XmlElement, XmlEntityReference, and XmlProcessingInstruction). Several of these key types also have further subclasses. In each case, the final subclass of each inheritance branch has a name that is meaningful to one familiar with XML.

Figure 1-3 shows the XmlNode inheritance hierarchy.

Figure 1-3. XmlNode inheritance hierarchy

Each of the concrete XmlNode subclasses are also represented by the members of the XmlNodeType enumeration: Element, Attribute, Text, CDATA, EntityReference, Entity, ProcessingInstruction, Comment, Document, DocumentType, DocumentFragment, Notation, Whitespace, and SignificantWhitespace, plus the special pseudo-node types, None, EndElement, EndEntity, and XmlDeclaration. Each XmlNode instance has a NodeType property, which returns an XmlNodeType that represents the type of the instance. An XmlNodeType value is also returned by the NodeType property of XmlReader, as discussed in Chapter 2, Chapter 3, and Chapter 4.