XML Processing

C# and Windows Programming

XML Processing

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Contents

Markup XML DTDs XML Parsers DOM

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Markup

When we write text, it is just text For example:

John Smith 123 Main St. Toronto Ontario

We can all read this and understand it A computer cannot and needs additional

information

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Markup

Markup is added to documents in the form of tags

A tag consists of text delimited by angle brackets

The name of the tag identifies it and the information which is conveyed by the tag

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Markup

Let’s add some semantic markup to our address<address>

<name>John Smith</name><street>123 Main St.</street><city>Toronto</city><province>Ontario</province>

</address> This identifies the information in the various

parts of the address

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Markup

You will notice Tags occur in pairs

A start tag A matching end tag with a “/” before the tag name

The text that the tags are describing is enclosed between the start tag and the end tag

A single tag is placed around the entire document The fact that every start tag has a matching end tag

makes the document well-formed

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XML

XML is one of in a long line of markup languages

It is the eXtensible Markup Language Unlike, other markup languages, you can

define your own tags Any meaning associated with those tags is

imposed by your program

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Uses of XML

SOAPSimple Object Access Protocol – a type of

remote procedure call Configuration files Web services Security information Electronic document exchange

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Defining Documents

If you can define your own tags, how do you know what should be in a document?Document Type Definition

This defines the allowable tags and their orderSchema

Like a DTD, it describes the tags and their order It also describes the content which can be placed

within the tags

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XML Structure

Here is a simple XML document<?xml version=“1.0” encoding=“ISO-8859-1”

standalone=“no”?><!DOCTYPE address SYSTEM “address.dtd”><address>

<name>John Smith</name><street>123 Main St.</street><city>Toronto</city><province>Ontario</province>

</address>

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Attributes

A tag can also have attributes which provide additional information about the tag<city size=“large”>Toronto</city>

A tag can have zero or more attributes

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The XML Declaration

The first line is the optional XML declaration

It consists of<?xml

Identify this as the XML declaration

version=“1.0” The version of XML in the document

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The XML Declaration

encoding=“ISO-8859-1” This is the character set used in the document Various character sets can be used including unicode (UTF-

8) an international character set

standalone = “no” Determines if the document uses any external entities which

are defined in other files This will be discussed later in the course

In general, the order of attributes is not important but it is in the XML declaration

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The DOCTYPE Declaration

The optional DOCTYPE declaration follows the XML declaration<!DOCTYPE address SYSTEM

“address.dtd”> This declaration is required only if you

want to validate the document against a definition of the tags in the document

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The Root Element

This is the <address> element which begins the document

It is the first element in the document It contains all other elements in the

document

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Elements

An element consists of a start tag, character data, and an end tag <name>John Smith</name>

A tag name must start with a letter or underscore A tag name cannot contain spaces or colons The end tag must match the start tag exactly,

including case

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Mixed Content

If an element contains just text, it has simple content<name>John Smith</name>

If it contains a mix of text and elements, it is said to have mixed content<sentence>these are nested

<adverb>correctly</adverb></sentence>

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Attributes

Attributes are name-value pairs which can be added to elements

Attributes allow you to provide additional information without changing the tag itself

The names for attributes follow the same rules as tag names

Every attribute name within the same tag must be unique

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Attributes

<employee name=“Jones”>accountant<employee><employee name=“Smith”>sales<employee>

Note that these both contain a name attribute

That is OK since the attributes are in separate elements

Attribute values are placed in either single or double quotes

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Comments

Comments are delimited by spacial bracketsComments can

Add explanations Remove XML which is not needed for a while

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Entities

The less than and greater than signs delimit tags What if you want to type these symbols in a document

and not have them delimit a tag? Then, enter them as entities To enter a less than sign

< All entities are referenced using

& The entity name ;

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Entities

Entity Symbol Description

< < Less than

> > Greater than

& & Ampersand

" “ Double quote

' ‘ apostrophe

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CDATA

Sometimes using entities is not enough since you have many special characters to type

A CDATA section allows you to enter anything without having special characters interpreted<![CDATA[ any characters here ]]>

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Document Type Definitions

The DTD is one way to describe what should be in a valid XML document

There are other ways which we will examine later in the course

A DTD Describes each element and the elements which can

occur within it Describes the attributes for each element Describes entities which can be used in the document

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Person DTD

<!DOCTYPE persontype [<!ELEMENT person (first, last, gender, employee-id) ><!ELEMENT first (#PCDATA) ><!ELEMENT last (#PCDATA) ><!ELEMENT gender (#PCDATA) ><!ELEMENT employee-id (#PCDATA) >

]>

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Reading the DTD

There is an element person containing the elements first last gender employee-id

These element are described below Each of these contains PCDATA, meaning parseable

character data This means that these elements only contain text – not

nested tags

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XML Parsers

There are two types of XML parsers DOM

The Document Object Model This parses the document into a tree-like structure called a

DOM The document is parsed all at once

SAX Simple Api for Xml This is a sequential parser which executes a callback when

each part of the document is recognized This is good for very large documents since the entire

document does not have to be in memory at once

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What is DOM?

DOM is an in-memory data structure It describes an XML document as a tree

structure The nodes in the tree are described by the

interface to them This means that there can be many

implementations that implement the interface

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So, how do make a document into a tree?<?xml version=“1.0”?>

<friend>

<handle degree=“close”>

Harold

</handle>

</friend>

Document

friend

whitespace handle

Harold

whitespacedegree

close

RootElement

Text

Attribute

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Nodes

All nodes in a DOM implement the Node interface

All other interfaces in the tree extend the Node interface

This means that every node can be treated as a Node, and maybe more

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XmlNode

Represents every node in the DOM Properties

ParentNodeNameFirstChildNextSiblingPreviousSiblingValue

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XmlNode

Methods InsertBefore()AppendChild()RemoveChild()Clone()

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XmlDocument

The node above the root node of the document Can be used to represent an empty document Properties

DocumentElement Methods

CreateElement() CreateTextNode() GetElementsByTagName() Load() Save()

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XmlElement

This represents an element An element can have attributes Properties

XmlAttributeCollection Attributes

Methods GetElementsByTagName() SetAttribute(string name, string value) string GetAttribute(string name)

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XmlAttribute

This is an attribute Can have either Text nodes or

EntityReferences as children Name property gets the name Value gets the value

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XmlText

This is the node representing text The text has no markup Even whitespace is represented as a text

node

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CDATASection Interface

This is a CDATA section It is similar to a text node but the content

undergoes no interpretation

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Other Node Subinterfaces

Comment Notation Entity EntityReference ProcessingInstruction

These are all just the same as in XML

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DocumentFragmentPart of a document tree which can be inserted

into another tree DOMImplementation

Prevides capabilities of the implementationHas the method for creating a document

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DOMExceptionSomething went wrong

NodeListA list of nodes which has an iterator

NamedNodeMapA map structure holding a collection of nodes

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Common .NET DOM Classes

XmlNode

XmlDocument XmlElement XmlText XmlAttribute

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XmlNodeList

A list of nodes Returned by GetElementsByTagName() Properties

Count -- number of nodes in the list Indexer -- retrieves a node

Methods Item(int n) -- retrieves a node

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XmlNamedNodeMap

A map of nodes indexed by name Superclass of XmlAttributeCollection Returned by the Attributes property Properties

Count

Methods Item(int n) GetNamedItem(string name)

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Examples

* see NodeLister * see DocBuilder