The Document Object Model Implementation

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The Document Object Model (DOM) is the representation of an XML document as a set of objects that can manipulate the structure and contents of the document. By now, you should be aware of this concept. Chapter 2 introduced the DOM, Chapter 3 gave examples of how to work with the DOM, and Chapter 5 went into detail about the DOM and how it is used with MSXML. This section details how the DOM is represented in .NET.

The DOM implementation in .NET is like the MSXML parser. The entire structure is loaded into memory, and the document's parts can be accessed as a set of linked nodes. These nodes are manipulated by using the XmlNode class and its derived classes. Refer to Figure 6.5, which shows the different implementations of the XmlNode abstract class. Notice that one of the classes derived from XmlNode is the XmlDocument class: This class forms the basis for the DOM implementation in .NET.

The XmlDocument Class

Loading the entire document into memory provides many benefits. For example, it's easy to navigate through the DOM, adding, updating, and removing nodes within it. We discussed how this functionality comes at a price: Loading the entire document requires more memory than reading the document as a stream, compounded by the fact that each user accessing the document creates their own object instance in memory. However, this is an acceptable tradeoff for most scenarios where the XML being loaded is relatively small.

Consider the Usage in Your Designs

As with all distributed application designs, you need to consider whether you need the flexibility of updating, adding, and removing nodes with the XmlDocument class, or if you only need to read the document with an XmlReader class.

Looking at the XmlDocument class shows that the methods and properties of the class are similar to those of the MSXML parser. Table 6.6 shows the properties of the XmlDocument class, Table 6.7 shows its methods, and Table 6.8 shows its events.

Table 6.6. Properties of the XMLDocument Class

Property Name	Description
Attributes	Gets an XmlAttributeCollection that contains the attributes of this node.
BaseURI	Gets the base URI of the current node. Indicates from where the node was loaded.
ChildNodes	Gets all the children of the current node.
DocumentElement	Retrieves the root element of the XML document as an XmlElement object.
DocumentType	Gets the <!DOCTYPE..> declaration, if one exists as an XmlDocumentType object.
FirstChild	Gets the first child of the current node.
HasChildNodes	Returns a Boolean that indicates if the current node has child nodes.
Implementation	Returns the XmlImplementation object for the XML document.
InnerText	Gets or sets the concatenated values of the node and all its children.
InnerXml	Gets or sets the markup that represents the children of the current node.
IsReadOnly	Returns a Boolean that indicates whether the current node is read only.
Item	Returns the specified child element. This is the indexer in C# for the XmlDocument class.
LastChild	The last child of the node.
LocalName	Gets the name of the node without the namespace prefix.
Name	Gets the qualified name of the node.
NamespaceURI	Returns the namespace URI of the current node.
NameTable	Returns the XmlNameTable associated with this implementation.
NextSibling	Returns the node following this node at the same level in the document hierarchy.
NodeType	Returns an XmlNodeType enumeration member that represents this node's type.
OuterXml	Gets the XML markup that represents the current node and its children. This is similar to InnerXml , except the current node is also returned.
OwnerDocument	Gets the XmlDocument to which the current node belongs.
ParentNode	Gets the parent of the current node. If the node has been created but not added to the tree, this returns a null reference in C#, or nothing in Visual Basic .NET.
Prefix	Gets or sets the namespace prefix for the current node.
PreserveWhitespace	Gets or sets a Boolean that indicates if white space would be preserved or stripped.
PreviousSibling	Gets the node previous to this node at the same level in the document hierarchy.
Value	Gets or sets the value of the current node.
XmlResolver	Sets the XmlResolver to use for resolving external resources.

Table 6.7. Methods of the XMLDocument Class

Method Name	Description
AppendChild	Adds the specified node to the end of the list of children of the current node.
Clone	Creates a duplicate of the current node.
CloneNode	Similar to Clone , but accepts a deep Boolean parameter that indicates if the subtree should be duplicated also.
CreateAttribute	Creates an XmlAttribute node with the specified name.
CreateCDATASection	Creates an XmlCDATASection that contains the specified data.
CreateComment	Creates an XmlComment that contains the data specified.
CreateDocumentFragment	Creates an empty XmlDocumentFragment used for tree inserts .
CreateDocumentType	Creates an XmlDocumentType object.
CreateElement	Creates an empty XmlElement . Uses the Value property to specify text data for the element or append an XmlTextNode as the element's child.
CreateEntityReference	Creates an XmlEntityReference with the specified name.
CreateNavigator	Creates an XpathNavigator for navigating this object as a DOM structure.
CreateNode	Creates an XmlNode object with the specified XmlNodeType .
CreateProcessingInstruction	Creates an XmlProcessingInstruction that contains the specified target and data. To work with the <?xml version="1.0"?> xml declaration, use the CreateXmlDeclaration method.
CreateSignificantWhitespace	Creates an XmlSignificantWhitespace node that contains only the characters &#20;, &#10;, &#13; , and &#9;.
CreateTextNode	Creates an XmlTextNode with the passed in text.
CreateWhitespace	Creates an XmlWhitespace node containing only the characters &#20;, &#10;, &#13; , and &#9; .
CreateXmlDeclaration	Creates an XmlDeclaration object with the specified values. Accepts parameters for version, encoding, and standalone.
GetElementByID	Gets the XmlElement with the specified ID attribute. If multiple matches are found, returns only the first match.
GetElementsByTagName	Returns an XmlNodeList that contains all the descendant elements with the specified name.
GetEnumerator	Enables For Each enumerations on the object.
GetHashCode	Serves as a hash function for a particular type, suitable for use in hashing algorithms and data structures, such as a hash table.
GetNamespaceOfPrefix	Retrieves the namespace URI in the closest XMLNS declaration for the given prefix.
GetPrefixOfNamespace	Retrieves the prefix in the closest XMLNS declaration for the given URI.
GetType	Gets the type of the current instance.
ImportNode	Imports a node from an external document to the current document.
InsertAfter	Inserts the node immediately after the specified reference node.
InsertBefore	Inserts the node immediately before the specified reference node.
Load	Loads the XML data. Sources for the XML data include a URL, a stream, a text reader, and an XmlReader .
LoadXml	Loads the XML document from the string parameter.
Normalize	Puts all XmlText nodes in the full depth of the subtree underneath this XmlNode into a normal form where only markup (for example, tags, comments, processing instructions, CDATA sections, and entity references) separates XmlText nodes; that is, no adjacent XmlText nodes exist.
PrependChild	Adds the specified node as the first child of the current node.
ReadNode	Creates an XmlNode object based on the information in the XmlReader . The reader must be positioned on a node or attribute. If positioned on an element node, it advances the reader to the next position. If positioned on an attribute, it does not advance the reader.
RemoveAll	Removes all children and/or attributes of the current node.
RemoveChild	Removes the specified child node associated with the current node.
ReplaceChild	Removes the old child node and replaces it with the new child node.
Save	Saves the XML document to the specified location.
SelectNodes	Selects an XmlNodeList of nodes that match the specified XPath expression.
SelectSingleNode	Selects an XmlNode that matches the specified XPath expression. If more than one match is found, only the first match is returned.
Supports	Tests if the DOM implementation supports a specific feature.
ToString	Returns a string that represents the current object.
WriteContentTo	Saves the contents of the XmlDocument node to the specified XmlWriter .
WriteTo	Saves the XmlDocument node to the specified XmlWriter .

Table 6.8. Events of the XMLDocument Class

Event Name	Description
NodeChanged	The value of a node belonging to this document has been changed.
NodeChanging	The value of a node belonging to this document is about to change.
NodeInserted	A node belonging to this document has been inserted into another node.
NodeInserting	A node belonging to this document is about to be inserted into another node.
NodeRemoved	A node belonging to this document has been removed.
NodeRemoving	A node belonging to this document is about to be removed.

Populating an XmlDocument

Several methods for loading an XmlDocument object with data exist. You've already seen examples of using a file to load the XmlDocument earlier in this chapter. You can also specify a string to populate it with data, as shown here:

 <%@ Import Namespace="System.Xml"%>  <%@ Import Namespace="System.Text"%>  <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" >  <HTML>       <HEAD>            <title>WebForm1</title>            <script language="C#" runat="server">            private void Page_Load(object sender, System.EventArgs e)            {                StringBuilder sb = new StringBuilder();              sb.Append("<DATA xmlns=\"urn:foo:bar\">");              sb.Append("<PERSON name=\"Carson Allen Evans\"/>");                 sb.Append("<PERSON name=\"Deanna Evans\">Hello, Wife</PERSON>");                 sb.Append("</DATA>");                 XmlDocument doc = new XmlDocument();                 doc.LoadXml(sb.ToString());                 TextBox1.Text = doc.OuterXml;           }            </script>    </HEAD>    <body>         <form runat="server">          <asp:TextBox id="TextBox1" runat="server" TextMode="MultiLine" Width="473px"  Height="447px"></asp:TextBox>         </form>       </body>  </HTML> 

This example uses a StringBuilder to build a string representation of the XML document. The XmlDocument object is then populated by calling the LoadXml method. You could also use an XmlTextReader to populate the XmlDocument because an XmlTextReader is an implementation of the XmlReader class.

Recall that you can use the XmlTextReader to access secured documents on external resources. Using the XmlTextReader as the source for the XmlDocument is a good choice in this situation.

Now that you have seen how to get data into the XmlDocument , take a look at how to navigate through its contents.

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