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XML 10-SECOND TUTORIAL: Objects and Interfaces
There are a number of different objects and interfaces
for XML---MSXML.DOMDocument, IDOMDocument, XMLDocument,
Microsoft.XMLDOM---but which object should you use with
Visual Basic and Internet Explorer?
The object library for XML can get a little confusing.
It's worth understanding the nomenclature to make
programming against the DOM easier. (DOM stands for
Document Object Model. Check out
http://www.xml.com/pub/r/Document_Object_Model_(DOM)_Requirements
for more info on the DOM.)
The principal progIDs (the classnames used in
CreateObject() calls) all start with Microsoft:
 | Microsoft.XMLDOM creates an XML document. |
| Microsoft.XMLHTTP creates an XML HTTP Request object. |
| MSXML.DOMDocument is the older form of Microsoft.XMLDOM
and shouldn't be used. |
All of the derived interfaces, or what VB programmers
would call a PublicNotCreateable object, start with the
letter "I" for interface:
| IXMLDOMElement is an element interface. |
| IXMLDOMDocument is a document interface. |
| IXMLDOMNodeList is a node-list interface. |
There's also a DOMDocument class, which is multi-use. It
implements the same interface as IDOMDocument, but
DOMDocument is a creatable class while IDOMDocument is a
derived class. Thus, this VB code creates a new document
and assigns an element to it:
Dim xmlDoc As New DOMDocument
Dim xmlNode As IXMLDOMElement
' Create Node Object
Set xmlNode=xmlDoc.CreateElement("document")
' Append Node Object
Set xmlDoc.AppendChild xmlNode
You should note that the IXMLDOMElement inherits from the
IXMLDOMNode interface. This is important when dealing
with Visual Basic because the IXMLDOMNode interface
doesn't support the .selectNodes or .selectSingleNode
methods.
The odd part is that selectSingleNode itself returns an
IXMLDOMNode object, but you can implicitly coerce it into
an IXMLDOMElement by simple assignment:
Dim newNode As IXMLDOMElement
Set newNode = xmlNode.selectSingleNode("//myNode")
The newNode here is an IXMLDOMElement node, while the
.selectSingleNode() function produces an IXMLDOMNode
object.
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