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Introduction to XML

 

Overview of Files

Consider the following list:

We have already seen how to create and save such a list. When you create this type of list, you choose the programming environment of your choice and save the file. You cannot open that file in just any application. You must create and distribute the application to those who want to be able to open a file created by your application. It could be useful to create a type of file using any application of your choice and be able to open that file using another type of application that you may not even know.

 

Introduction to XML

The Extensible Markup Language, or XML, is a technique of using a document, such as a text file, to describe information and make that information available to whatever and whoever can take advantage of it. The description is done so the document can be created by one person or company and used by another person or another company without having to know who first created the document. This is because the document thus created is not a program, it is not an application: it is just a text-based document.

Because XML is very flexible, it can be used in regular Windows applications, in databases, in web-based systems (Internet), in communication applications, in computer networks, in scientific applications, etc. To make sure that XML can be universally used without one person or group owning it, it is standardized by the W3C (http://www.w3c.org) organization. XML is released through an XML Recommendation document with a version.

In our lessons, we will learn or use XML through the .NET Framework classes. The particularity is that these classes are highly structured to take care of all facets of XML without compromising the standards. In fact, the .NET Framework classes are highly conform to the W3C standards in all areas.

To create an XML file, in the document, you type units of code using normal characters of the English language. The XML document is made of units called entities. These entities are spread on various lines of the document as you judge them necessary and as we will learn. XML has strict rules as to how the contents of the document should or must be structured.

After an XML document has been created and is available, in order to use it, you need a program that can read, analyze, and interpret it. This program is called a parser. The most popular parser used in Microsoft Windows applications is MSXML, published by Microsoft.

Practical Learning Practical Learning: Introducing XML

  • Start Microsoft Visual C# and create a Windows Application named RedOakHighSchool1

Markup

A markup is an instruction that defines XML. The fundamental formula of a markup is:

<tag>

The left angle bracket "<" and the right angle bracket ">" are required. Inside of these symbols, you type a word or a group of words of your choice, using regular characters of the English alphabet and sometimes non-readable characters such as ?, !, or [. The combination of a left angle bracket "<", the right angle bracket ">", and what is inside of these symbols is called a markup. There are various types of markups we will learn.

The Document Type Declaration (DTD)

As mentioned above, XML is released as a version. Because there can be various versions, the first line that can be processed in an XML file must specify the version of XML you are using. At the time of this writing, the widely supported version of the .NET Framework is 1.0. When creating an XML file, you should (should in 1.0 but must in 1.1) specify what version your file is conform with, especially if you are using a version higher than 1.0. For this reason, an XML file should start (again, must, in 1.1), in the top section, with a line known as an XML declaration. It starts with <?xml version=, followed by the version you are using, assigned as a string, and followed by ?>. An example of such a line is:

<?xml version="1.0"?>

By default, an XML file created using Visual Studio 2005 specifies the version as 1.0. Under the XML declaration line, you can then create the necessary tags of the XML file.

Encoding Declaration

As mentioned already, the tags are created using characters of the alphabet and conform to the ISO standard. This is known as the encoding declaration. For example, most of the characters used in the US English language are known as ASCII. These characters use a combination of 7 bits to create a symbol (because the computer can only recognize 8 bits, the last bit is left for other uses). Such an encoding is specified as UTF-8. There are other standards such as UTF-16 (for wide, 2-Byte, characters).

To specify the encoding you are using, type encoding followed by the encoding scheme you are using, which must be assigned as a string. The encoding is specified in the first line. Here is an example:

<?xml version="1.0" encoding="utf-8"?>

Creating an XML File

Due to the high level of support of XML in the .NET Framework, there are various ways you can create an XML file. The most common technique consists of using a simple text editor. In Microsoft Windows, this would be Notepad. An XML file is first of all a normal text-based document that has a .xml extension. Therefore, however you create it, it must specify that extension.

Many other applications allow creating an XML file or generating one from an existing file. There are also commercial editors you can get or purchase to create an XML file.

Practical Learning Practical Learning: Creating an XML File

  1. On the main menu, click File -> New -> File
  2. In the New File dialog box, click XML File and click Open.
    Notice that the file already contains the encoding

Writing XML Code

 

Introduction to the Document Object Model

To implement XML, the .NET Framework provides the System.Xml namespace. When you create an XML file, there are standard rules you should (must) follow in order to have a valid document. The standards for an XML file are defined by the W3C Document Object Model (DOM). To support these standards, the System.Xml namespace provides the XmlDocument class. This class allows you to create an XML document, to populate it with the desired contents, and to perform many other related operations on the contents of the file.

Here is an example:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Xml;

namespace MusicCollection
{
    public partial class Exercise : Form
    {
        public Exercise()
        {
            InitializeComponent();
        }

        private void btnDocument_Click(object sender, EventArgs e)
        {
            XmlDocument docMusicCollection = new XmlDocument();
        }
    }
}

Writing XML Code Using XmlDocument

To create XML code using XmlDocument, the class has a method called LoadXml(). Its syntax is:

public virtual void LoadXml(string xml);

This method takes a string as argument. The XmlDocument.LoadXml() method doesn't create an XML file, it only allows you to provide or create XML code. The code can be created as argument. You can also first declare and initialize a string variable with the XML code, then pass it as argument to the XmlDocument.LoadXml() method.

Writing XML Code Using the Code Editor

In the next sections, we will see how to create an XML file. After creating the file and displaying it in the Code Editor, you can start editing it. The Code Editor is equipped to assist you with various options. Whenever you start typing, the editor would display one or more options to you:

XML Code

When different options are presented to you, you can press the arrow keys to select an option and press Enter. You can also use the mouse to click an option. When typing other XML items as we will learn, the Code Editor is equipped to work in concert with you.

 

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