The XmlNode Type Hierarchy

Because the .NET Framework provides a complete Level 2 Core implementation, the standard node inheritance tree is available. As you’ll recall from Chapter 1, each node in an XML document is represented by an appropriately named class, starting with the abstract base class, XmlNode. Look at Figure 5-2 and compare the XmlNode inheritance hierarchy to the DOM Node inheritance hierarchy in Figure 5-1. You should see that every DOM type maps to exactly one .NET XmlNode subclass, although some XmlNode subclasses do not have an equivalent DOM type.

XmlNode inheritance hierarchy
Figure 5-2. XmlNode inheritance hierarchy

You can also see in Figure 5-2 that the .NET Framework inserts some additional levels of inheritance in the DOM hierarchy. These additional types provide a place for groupings of common functionality (XmlLinkedNode) as well as adding some functionality that is not required by the DOM specification (XmlWhitespace, XmlSignificantWhitespace).

The .NET DOM implementation provides intuitive names, similar enough to the relevant DOM interface to understand without further comment. In most cases, there is a one-to-one relationship between a DOM interface and the .NET implementation; however, Table 5-1 lists the exceptions to that rule.

Table 5-1. .NET DOM implementation exceptions

DOM interface

.NET implementation

Notes

DOMString
 
String

The DOM recommendation specifies that a language binding may use any type to represent a DOMString, as long as it supports UTF-16 encoding.

DOMTimeStamp

None

The DOM recommendation specifies that a language binding may use any type to represent a DOMTimeStamp, as long as it can hold the time in milliseconds. However, the Core module does not actually specify any use for the DOMTimeStamp type.

DOMException
 
XmlException

A DOMException is only raised when an error prevents operations from continuing. XmlException signals these conditions as well as framework-specific errors.

ExceptionCode

None

The ExceptionCode is not directly implemented in .NET. The DOM recommendation specifies that ExceptionCode is only necessary in language bindings that do not support exceptions natively.

DOMImplementation
 
XmlImplementation

.NET includes the CreateDocumentType( ) method on the XmlDocument class. The createDocument( ) and createDocumentType( )methods were added to DOMImplementation in DOM Level 2.

Document
 
XmlDocument

.NET adds the Document constructor and includes the CreateDocumentType( ) factory method here rather than on DOMImplementation.

Node
 
XmlNode

The DOM Level 2 methods isSupported( ) and hasAttributes( ) are not implemented.

NodeType
 
XmlNodeType

The DOM NodeType defines a list of codes for the various node types; the XmlNodeType enum defines six additional types to allow it to be used by other classes.

NodeList
 
XmlNodeList

XmlNodeList implements the IEnumerable interface, therefore individual nodes can be accessed via the standard IEnumerator methods.

NamedNodeMap
 
XmlNamedNodeMap

XmlNamedNodeMap implements the IEnumerable interface, therefore individual nodes can be accessed via the standard IEnumerator methods.

Tip

IEnumerable and IEnumerator are .NET interfaces that define methods that are used to move through a collection of objects. They are not DOM interfaces, although they are used in .NET DOM processing.

Another general exception is that when the DOM interface has a method named xxxNS( ), the corresponding .NET Xxx( ) method is simply overloaded to include the namespace URI parameter. The DOM interfaces are specified this way because IDL does not support overloaded methods.

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