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XML Basics

What is XML?

• XML means Extensible Markup Language
   
• XML is
   
  
  a data storage and transmission language
   
  simple, portable, and human readable
   
  hierarchical
   
  self documenting (unlike fixed field or CSV files)
   
  positioned to be the linga franca of data
  
  
• .Net uses XML as its preferred internal data representation format.

CSV vs. XML

• Compare these CSV and XML files containing the same data:
   
  CSV Version

  Jack,38
  Tom,36
  

  XML Version
  

  <NineHoleGolfScores>
     <Player ID="348">
        <Name>Jack</Name>
        <Score>38</Score>
     </Player>
     <Player ID="541">
        <Name>Tom</Name>
        <Score>36</Score>
     </Player>
  </NineHoleGolfScores>
  

• XML is wordy but self documenting.

XML Definitions

• An element is delimited by angle brackets.
  Example:<Name> or </Name>
   
• A start tag is delimited by < ... > with no /.
   
• An end element is delimited by </ ... >
  Example:</Name>
   
• Every start tag has a matching end tag with the same name.
   
• A start tag may contain additional information called attributes.
  In this example, ID is the attribute: <Employee ID="1291">
   
• An XML document consists of a tree of elements of various types.
   
• Examples of XML elements:
   
  
  A declaration element contains version and validation information for the XML document. Example:
  

  <?xml version="1.0" standalone="no"?>
  

  An empty element contains only a start tag and an end tag:
  

  <FirstName></FirstName>
  

  Alternative syntax for an empty element:
  

  <FirstName />
  

  An element with text:
  

  <FirstName>Jerry</FirstName>
  

  /An element with an attribute and text:
  

  <Employee ID="2354">Jerry</Employee>
  

  A nested element:
  

  <Singer ID = 240>
  <FirstName>Madonna</FirstName>
  <LastName />
  </Singer>
  

  Whitespace Elements:
   
  
  Space   Tab   Line Feed   Carriage Return  
  
  Comment Element:
  

  <!-- This is a comment -->
  

  The root element contains all of the data in an XML file. In the Persons.xml file, Persons is the root element.
   
  An XML file can only contain one root element.

Checking XML Files for Correctness

• Nonvalidating Parsers
   
  
  Checks if XML file is a well formed document according to W3C specifications.
  
  
• Validating Parsers
   
  
  Checks the correctness of XML document against a DTD (Document Type Definition) or XSD (XML Schema Definition) file.
   
  A Schema can be created in .Net.
  
  
• Visual Studio can check an XML file for errors. Open an XML file using the Visual Studio Editor. If the file is not a legal XML document, the errors will be indicated. Try it out on this Persons.xml file.

XML Forward-only Processing

• With forward-only processing, XML elements are processed sequentially.
   
• No backtracking is allowed.
   
• Can be done with an XMLTextReader.