Reading and Writing XML Documents

One of the most basic tasks that you can perform with XML is manipulating the contents of an XML document. This includes traversing the list of nodes in the document, setting and querying attribute values, and manipulating the tree itself by creating and inserting new nodes.

This section shows you how to read XML documents using the Document Object Model (DOM) ) modeled by the XmlDocument class in the System.Xml namespace. The DOM is recursive, meaning that each node has the same properties and methods as every other node in the document. Tables 8.1 and 8.2 provide a brief overview of many of the properties and methods of the XmlNode class before getting into the code sample. The XML node is the most basic unit of abstraction within a DOM-modeled document.

Table 8.1. Commonly Used XmlNode Properties

Property	Method
Attributes	The attributes of the node (XmlAttributeCollection).
ChildNodes	The list of child nodes of the current node (XmlNodeList).
FirstChild	Returns the first child of the XML node (first being first in document order).
HasChildNodes	A Boolean that indicates whether the node has child nodes.
InnerText	Gets or sets the text inside the node.
InnerXml	Gets or sets the XML within the node.
LastChild	Returns the last child (document order relative) of the node.
Name	The name of the node.
NodeType	Indicates the type of the node. This can be several things, including (but not limited to): Document, DocumentFragment, Element, EndElement, Entity, Notation, Text, Whitespace, or XmlDeclaration.
OuterXml	The XML representing the current node and all its child nodes.
OwnerDocument	The document to which the current node belongs.
ParentNode	The parent node of the current node, if any.
PreviousSibling	Gets the node immediately preceding the current node in document order.
Value	Gets or sets the value of the current node.

Table 8.2. Commonly Used XmlNode Methods

Method	Description
AppendChild	Adds a child node to the end of the current list of child nodes
Clone	Creates a duplicate of the node
CreateNavigator	Creates an XPathNavigator for this node
InsertAfter	Inserts the given node immediately after the current node
InsertBefore	Inserts the given node immediately before the current node
PrependChild	Adds the given child node at the beginning of the child node list
RemoveAll	Removes all child nodes
RemoveChild	Removes the given child from the current node
SelectNodes	Selects a list of nodes matching the XPath expression (discussed in the following section)
SelectSingleNode	Selects a single node that matches the XPath expression

The XmlDocument class, which deals with the entire document, is also itself an XmlNode. If you look at the documentation for the XmlDocument class, you'll see that it inherits from XmlNode. This fits with the DOM pattern in that the document is a node that can have child nodes. The methods in Table 8.3 show some of the additional methods available to the XmlDocument that aren't part of a standard node class.

Table 8.3. XmlDocument Class Methods

Method	Description
CreateAttribute	Creates an XmlAttribute with the given name
CreateCDataSection	Creates a CData section with the given data
CreateComment	Creates an XmlComment
CreateDocumentFragment	Creates a document fragment
CreateElement	Creates an XmlElement, an XmlNode with element-specific functionality
CreateNode	Creates an XmlNode
CreateTextNode	Creates an XmlText node
CreateWhitespace	Creates whitespace for insertion into the document
ImportNode	Imports a node from another document
Load	Loads the XML from the given file
LoadXml	Loads the XML from the given XML string
Save	Saves the XML document
Validate	Validates the XML document against a schema

To show off the code for manipulating an XML document, we need a document. For the rest of this chapter, we will be working with a document called items.xml, which contains a list of items from a fictitious role-playing game and the various aspects of those items. The contents of this file are shown in Listing 8.1.

Listing 8.1. Items.xml

<?xml version="1.0" encoding="utf-8"?> <items>   <item  name="Flaming Sword of Doom"         description= "This sword will vanquish all of your foes with a single swipe">     <attribute name="attack" value="10"/>     <attribute name="weight" value="21" />   </item>   <item  name="Bag of Really Big Stuff"         description="This bag can hold a lot of stuff.">     <attribute name="weight" value="1" />     <attribute name="capacity" value="80" />   </item>   <item  name="Broach of Bug Smashing"         description="This broach will kill any bug. Instantly.">     <attribute name="weight" value="1" />     <attribute name="attack" value="11" />     <specials>       <special name="killbug" description="This thing kills any bug instantly."/>     </specials>   </item>   <item  name="Wand of Traffic Vanquish"         description= "A single wave of this wand will part the highway before you.">     <attribute name="weight" value="5" />     <attribute name="attack" value="20" />     <specials>       <special name="parttraffic" description="All nearby vehicles move out of your way." />     </specials>   </item> </items> 

Although XML allows you to freely mix freeform text with markup, for the purposes of the code in this chapter, the examples use XML documents as pure data storage.

The code in Listing 8.2 shows the use of the XmlNode class and its associated properties and methods for traversing an XML document and displaying its contents to the console.

Listing 8.2. XML Document Display Sample

using System; using System.Xml; using System.Collections.Generic; using System.Text; namespace ReadWriteXml { class Program { static void Main(string[] args) { XmlDocument itemDoc = new XmlDocument(); itemDoc.Load(@"..\..\..\..\items.xml"); Console.WriteLine("DocumentElement has {0} children.",     itemDoc.DocumentElement.ChildNodes.Count); // iterate through top-level elements foreach (XmlNode itemNode in itemDoc.DocumentElement.ChildNodes) {     // because we know that the node is an element, we can do this:     XmlElement itemElement = (XmlElement)itemNode;     Console.WriteLine("\n[Item]: {0}\n{1}", itemElement.Attributes["name"].Value,         itemElement.Attributes["description"].Value);     if (itemNode.ChildNodes.Count == 0)         Console.WriteLine("(No additional Information)\n");     else     {         foreach (XmlNode childNode in itemNode.ChildNodes)         {             if (childNode.Name.ToUpper() == "ATTRIBUTE")             {                 Console.WriteLine("{0} : {1}",                     childNode.Attributes["name"].Value,                     childNode.Attributes["value"].Value);             }             else if (childNode.Name.ToUpper() == "SPECIALS")             {                 foreach (XmlNode specialNode in childNode.ChildNodes)                 {                     Console.WriteLine("*{0}:{1}",                        specialNode.Attributes["name"].Value,                        specialNode.Attributes["description"].Value);                 }             }         }     } } Console.ReadLine(); } } }