Applied XML Programming for Microsoft .NETPART 1

The .NET XML Parsing Model1. XML is a natural element of all forms of programming life.

2. XML in the .NET Framework

The .NET Framework XML core classes can be categorized according to their functions:

1. reading and writing documents

2. validating documents

3. navigating and selecting nodes

4. managing schema information

5. performing document transformations

The assembly in which the whole XML .NET Framework is implemented is system.xml.dll

The most commonly used namespaces are listed here:

1. System.Xml

2. System.Xml.Schema

3. System.Xml.XPath

4. System.Xml.Xsl

The .NET Framework also provides for XML object serialization.

The classes involved with this functionality are grouped in the System.Xml.Serializationnamespace.

XML serialization writes objects to, and reads them from, XML documents.

This kind of serialization is particularly useful over the Web in combination with the Simple Object Access Protocol (SOAP) and within the boundaries of .NET Framework XML Web services.

Areas of the .NET Framework in Which XML Is Key

Category Description

ADO.NET Data container objects (for example, the DataSetobject) are always transferred and remoted via XML

Configuration Application settings are stored in XML files, making use of predefined and user-defined section readers.

Remoting Remote .NET Framework objects can be accessed by using SOAP packets to prepare and perform the call.

Web services SOAP is a lightweight XML protocol that Web services use for the exchange of information in a decentralized, distributed environment.

XML parsing The core classes providing for XML parsing andmanipulation through both the stream-based API and the XML Document Object Model (XMLDOM).

XML serialization Supplies the ability to save and restore living instances of objects to and from XML documents

Classes for ParsingThe available XML parsers fall into one of two main categories:

1. tree-based parsers

2. event-based parsers

XML and ADO.NET

The interaction between ADO.NET classes and XML documents takes one of two

forms:

Serialization of ADO.NET objects (in particular, the DataSet object) to

XML documents and corresponding deserialization. Data can be saved to

XML in a variety of formats, with or without schema information, as a full

snapshot of the in-memory data including pending changes and errors, or

with just the current instance of the data

A dual-access model that lets you access and update the same piece of data either through a

hierarchical programming interface or using the ADO.NET relational API. Basically, you can

transform a DataSet object into an XMLDOM object and view the XMLDOM's subtrees as tables

merged with the DataSet object's tables.

The .NET Framework XML APIThe essence of XML in the .NET Framework is found in two abstract classes—

XmlReader and XmlWriter. These classes are at the core of all other .NET Framework

XML classes, including the XMLDOM classes, and are used extensively by various

subsystems to parse or generate XML text. For example, ADO.NET data adapters

retrieve the data to store in a DataSet object using a database reader, and the DataSet

object serializes its contents to the DiffGram format using an XmlTextWriter object,

which derives from XmlWriter

The XML API for the .NET Framework comprises the following set of functionalities:

1. XML readers

2. XML writers

3. XML document classes

Streams can be read and written using made-to-measure reader and writer classes.

The base classes are TextReader, TextWriter, BinaryReader, BinaryWriter, and

Stream. With the exception of the binary classes, all of these classes are marked as

abstract (MustInherit, if you speak Visual Basic) and cannot be directly instantiated in

code. You can use abstract classes to reference living instances of derived classes,

however.

In the .NET Framework, base reader and writer classes find a number of concrete

implementations, including StreamReader and StringReader and their writing

counterparts.

XML ReadersAn XML reader makes externally available a programming interface through which

callers can connect and pull out all the data they need. This is in no way different from

what happens when you connect to a database and fetch data. The database server

returns a reference to an internal object—the cursor—which manages all the query

results and makes them available on demand. This statement applies regardless of the

fact that the database world might provide several flavors of cursors—client, scrollable,

server-side, and so on.

Readers vs. XMLDOMXML readers don't require you to keep more data in memory than you actually need.

When you open the XML document, a simple logical pointer that corresponds to a node

is returned. You can easily skip over nodes to locate the one you need. In doing so, you

don't tax in any way the application's memory with extra data other than that required to

bufferize the currently selected node.

Readers vs. SAXA SAX parser directly controls the evolution of the parsing process and pushes data to

the client application. A cursor parser (that is, an XML reader), on the other hand, plays

a more passive role and leaves client applications to control the process

XML WritersThe .NET XML API separates parsing from editing and writing and offers a set of

methods that provides effective results for performance as well as usability. When

writing, you create new XML documents working at a considerably high level of

abstraction and explicitly indicate the XML elements to create—nodes, attributes,

comments, or processing instructions. The writer works on a stream, dumping content

incrementally, one node after the next, without the random access capabilities of the

XMLDOM but also without its memory footprint.

The XML Document Object API in .NETAs mentioned, along with XML readers and writers, the .NET Framework also provides

classes that load and edit XML documents according to the W3C DOM Level 1 and

Level 2 Core. The key XMLDOM class in the .NET Framework is XmlDocument—not

much different from the DOMDocument class, which you might recognize from working

with MSXML

XPath Expressions and XSLTIn the .NET Framework, XSLT and XPath expressions are fully supported but are

implemented in classes distinct from those that parse and write XML text. This is a key

feature of the overall .NET XML API. Any functionality is provided through a small

hierarchy of objects, although each subtree connects and interoperates well with

others.

The XMLDOM API is built on top of readers and writers, but both XSLT and

XPath expressions need to have a complete and XMLDOM-based vision of the entire XML

document to process it.

XML readers and writers are the primitive elements of the .NET XML API. Whenever

XML text must be parsed or written, all classes, directly or indirectly, refer to them. A

more complex primitive element is the XMLDOM tree. Transformations and advanced

queries must rely on the document in its entirety being held in memory and accessible

through a well-known interface—the XMLDOM.

The XSLT Processor

The key class for XSLT is XslTransform. The class works as an XSLT processor and

complies with version 1.0 of the XSLT recommendation. The class has two key

methods, Load and Transform, whose behavior is for the most part selfexplanatory

The XPath Query EngineXPath is a language that allows you to navigate within XML documents. Think of XPath

as a general-purpose query language for addressing, sorting, and filtering both the

elements and the text of an XML document.

Further ReadingFurther Reading

1.The W3C organization is currently working on a draft of the DOM Level 3 Core to

include support for an abstract modeling schema and I/O serialization. Check out the

most recent draft at http://www.w3.org/TR/2002/WD-DOM-Level3-ASLS-20020409. The

approved standard—DOM Level 2 Core—is available at http://www.w3.org/TR/DOMLevel-

2.Relevant information about XML standards is available from the W3C Web site, at

http://www.w3.org. If you want to learn more about the SAX specification, look at the

new Web site for the SAX project, at http://www.saxproject.org.