The Extensible Markup Language

Overview of Files

Consider the following list:

In Lesson 33, we saw how to create and save such a list. Here is an example of performing these operations:

using System;
using System.IO;
using System.Collections;
using System.Runtime.Serialization.Formatters.Binary;

[Serializable]
public sealed class Employee
{
    public string FirstName;
    public string LastName;
    public DateTime DateHired;
    public double HourlySalary;
}

public static class Program
{
    static int Main(string[] args)
    {
        ArrayList lstEmployees = new ArrayList();
        Employee contractor = null;

        contractor = new Employee();
        contractor.FirstName = "Johnny";
        contractor.LastName = "Watts";
        contractor.DateHired = new DateTime(1984, 5, 10);
        contractor.HourlySalary = 22.85;
        lstEmployees.Add(contractor);

        contractor = new Employee();
        contractor.FirstName = "Patrick";
        contractor.LastName = "Garçon";
        contractor.DateHired = new DateTime(2000, 7, 24);
        contractor.HourlySalary = 14.50;
        lstEmployees.Add(contractor);

        contractor = new Employee();
        contractor.FirstName = "Ernest";
        contractor.LastName = "Simms";
        contractor.DateHired = new DateTime(1996, 4, 16);
        contractor.HourlySalary = 18.05;
        lstEmployees.Add(contractor);

        contractor = new Employee();
        contractor.FirstName = "Gertrude";
        contractor.LastName = "Monay";
        contractor.DateHired = new DateTime(2002, 3, 8);
        contractor.HourlySalary = 26.35;
        lstEmployees.Add(contractor);

        FileStream fsEmployees = new FileStream("Employees.mpl",
                                                FileMode.Create,
                                                FileAccess.Write);
        BinaryFormatter bfEmployees = new BinaryFormatter();
        bfEmployees.Serialize(fsEmployees, lstEmployees);
        fsEmployees.Close();

        return 0;
    }
}

When saving this file, we chose an extension (.mpl) at random. After saving the values to a file, at one time, you may want to retrieve the values from that list. Here is an example:

using System;
using System.IO;
using System.Collections;
using System.Runtime.Serialization.Formatters.Binary;

[Serializable]
public sealed class Employee
{
    public string FirstName;
    public string LastName;
    public DateTime DateHired;
    public double HourlySalary;
}

public static class Program
{
    static int Main(string[] args)
    {
        FileStream fsEmployees = new FileStream("Employees.mpl",
                                                FileMode.Open,
                                                FileAccess.Read);
        BinaryFormatter bfEmployees = new BinaryFormatter();
        ArrayList contractors = (ArrayList)bfEmployees.Deserialize(fsEmployees);
        fsEmployees.Close();

        Console.WriteLine("======================================");
        Console.WriteLine("==== Employees Records ===============");
        foreach (Employee empl in contractors)
        {
            Console.WriteLine("--------------------------------------");
            Console.WriteLine("Employee Name: {0}, {1}",
                              empl.LastName, empl.FirstName);
            Console.WriteLine("Date Hired:    {0:D}", empl.DateHired);
            Console.WriteLine("Hourly Salary: {0:C}", empl.HourlySalary);
        }
        Console.WriteLine("======================================\n");

        return 0;
    }
}

This would produce:

======================================
==== Employees Records ===============
--------------------------------------
Employee Name: Watts, Johnny
Date Hired:    Thursday, May 10, 1984
Hourly Salary: $22.85
--------------------------------------
Employee Name: Garçon, Patrick
Date Hired:    Monday, July 24, 2000
Hourly Salary: $14.50
--------------------------------------
Employee Name: Simms, Ernest
Date Hired:    Tuesday, April 16, 1996
Hourly Salary: $18.05
--------------------------------------
Employee Name: Monay, Gertrude
Date Hired:    Friday, March 08, 2002
Hourly Salary: $26.35
======================================

Press any key to continue . . .

When you create this type of file, 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.

In this ebook, 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.

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.

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.

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"?>

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.

1. Start Microsoft Visual C# and create a Console Application named CollegeParkAutoParts1
2. To save the application, on the Standard toolbar, click the Save All button
3. Accept all defaults and click Save

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.

To create XML code using XmlDocument, this 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.

1. To start with XML, change the Program.cs file as follows:
    

   using System; using System.Xml; namespace CollegeParkAutoParts1 { class Program { static int Main(string[] args) { XmlDocument docXML = new XmlDocument(); docXML.LoadXml(""); return 0; } } }

2. Save the Program.cs file

In the next sections, we will see how to create an XML file with the Add New Item dialog box. 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. Here is an example:

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.

Probably the most common way to create an XML file in Microsoft Windows consists of using Notepad or any other text editor. After opening the text editor, you can enter the necessary lines of text. After creating the file, you must save it. When saving it, you can include the name of the file in double-quotes:

You can also first set the Save As Type combo box to All Files and then enter the name of the file with the .xml extension.

To assist you with creating XML Files, Microsoft Visual C# includes an XML File option in the Add New Item dialog box. After selecting this option, you can accept the suggested name of the file or replace it in the Name text box. If you don't specify the extension, the wizard would add it for you.

1. On the main menu, click Project -> Add New Item...
2. In the Templates list, click XML File
3. Set the Name to Parts
    
   
4. Click Add

If you call the XmlDocument.LoadXml() method, only the XML code is created, not the file. To actually create the Windows file, you can call the XmlDocument.Save() method. This method is provided in four versions. One of the versions takes as argument a string value. The syntax of this method is:

public virtual void Save(string filename);

The argument must be a valid filename and must include the .xml extension. If you pass a string without backlashes, the file would be created in the same folder as the current project. If you want the file to be created somewhere else (in a different directory), pass the whole path.

Whether you created an XML file or someone else did, you can open it easily to view its contents. The easiest way to open an XML file is to use a text editor, such as Notepad. Because the Code Editor has a friendlier appearance and it is available to you, it is a better candidate. To open an XML file, on the main menu, you can click File -> Open File..., locate the file, and click Open.

After creating an XML file, one way you can use it is to display it in a grid-based window such as a spreadsheet. Another way you can display an XML file is in a browser. To do this, if you see the file in Windows Explorer or in My Documents, you can double-click it. Here is an example:

When an XML file is displayed in a browser, it appears in a format that could be unclear. If you want the XML code to display as if it were HTML, you can create a cascading style sheet that would format the tags and display the text as you prefer. Here is an example: 

EmplNumber
{
	display: block;
	font-weight: bold;
	font-size: 16pt;
	color: white;
	font-family: Garamond, Georgia, 'Times New Roman' , Serif;
	background-color: #990000;
}
FirstName
{
	font-size: 10pt;
	font-family: Verdana, Tahoma, Arial, Sans-Serif;
	background-color: white;
}
LastName
{
	font-size: 10pt;
	font-family: Verdana, Tahoma, Arial, Sans-Serif;
	background-color: white;
}
HourlySalary
{
	font-size: 10pt;
	color: #ff0066;
	font-family: Verdana, Tahoma, Arial, Sans-Serif;
	background-color: white;
	display: block;
}

Then, in the first line of the XML file, you can add a line such as the following:

<?xml:stylesheet href="employee.css" type="text/css" ?>

At times, you will need to programmatically access an XML file. To support this operation, the XmlDocument class provides the Load() method which is available in various versions. One of the syntaxes used by this method is:

public virtual void Load(string filename);

This version takes as argument the name or path of the file. Here is an example of calling it:

using System;
using System.Xml;

namespace VideoCollection1
{
    class Program
    {
        static int Main(string[] args)
        {
            XmlDocument xmlDoc = new XmlDocument();

            xmlDoc.Load("Videos.xml");

            return 0;
        }
    }
}

In this case, the compiler would look for the file in the (Release) folder of the current application. You can also provide a complete path to the file. Either way, if the compiler doesn't find the file, it would throw a FileNotFoundException exception. For this reason, it is cautious to first check that the file exists before opening it. This can be done as follows:

using System;
using System.IO;
using System.Xml;

namespace VideoCollection1
{
    class Program
    {
        static int Main(string[] args)
        {
            XmlDocument xmlDoc = new XmlDocument();
            string strFilename = "Videos123.xml";

            if (File.Exists(strFilename))
                xmlDoc.Load(strFilename);
            else
                Console.WriteLine("The file {0} could not be located",
                                  strFilename);

            return 0;
        }
    }
}

An alternative is to handle the exception yourself.

You can also use a Stream-based object to identify the file. Once the object is ready, you can use the following version of the Load() method to open it:

public virtual void Load(Stream inStream);

This method expects a Stream type of object, such as a FileStream variable. Here is an example of calling it:

using System;
using System.IO;
using System.Xml;

namespace VideoCollection1
{
    class Program
    {
        static int Main(string[] args)
        {
            string strFilename  = "Videos.xml";
            XmlDocument xmlDoc  = new XmlDocument();
            FileStream fsVideos = null;

            if (File.Exists(strFilename))
            {
                fsVideos = new FileStream(strFilename,
                                                 FileMode.Open,
                                                 FileAccess.Read);

                xmlDoc.Load(fsVideos);
            }
            else
                Console.WriteLine("The file {0} could not be located",
                                  strFilename);

            return 0;
        }
    }
}

Many of the XML files you encounter will have been created by someone else. Still, because it is primarily a text document, you are expected to be able to read any XML file and figure out its content, or at least most of it. As mentioned already, you can open an XML file using a text editor such as Notepad. After opening the file, you can start by checking the document declaration, then move to other items.

Another way you can explore an XML file consists of programmatically reading it. This is also referred to as parsing (the parser parses the document). To support reading an XML file, the .NET Framework provides the abstract XmlReader class as the ancestor of classes that can read an XML file. One of the classes derived from XmlReader is called XmlTextReader. The XmlTextReader class provides the ability to read the file from the left to the right sides and from the top to the bottom sections. This class has very important characteristics you must remember:

• This class proceeds with a forward only approach: it reads a character or a word, moves to the next, and cannot refer back. This means that, after it has visited an item and has moved ahead, you cannot use it to refer to an item back
• This class is read-only: you cannot use it to edit or modify an item (but, when you get to an item, you can use another object to modify the item)

To programmatically read an XML file, you can start by declaring a variable of type XmlTextReader using one of its constructors, including the default. To specify the file you want to open and read, you can use the constructor whose syntax is the following :

public XmlTextReader(string url);

When using this method, pass the name of the file or its path as argument. Here is an example:

using System;
using System.IO;
using System.Xml;

namespace VideoCollection1
{
    class Program
    {
        static int Main(string[] args)
        {
            string strFilename = "Videos.xml";

            if (File.Exists(strFilename))
                XmlTextReader rdrXml = new XmlTextReader(strFilename);
            else
                Console.WriteLine("The file {0} could not be located",
                                  strFilename);

            Console.WriteLine();
            return 0;
        }
    }
}

You can also identify a file using a Stream-based object. Once the object is ready, you can pass it to the following constructor of the class:

public XmlTextReader(Stream input);

Here is an example:

using System;
using System.IO;
using System.Xml;

namespace VideoCollection1
{
    class Program
    {
        static int Main(string[] args)
        {
            string strFilename = "Videos.xml";
            FileStream fsVideos = new FileStream(strFilename,
                                                 FileMode.Open,
                                                 FileAccess.Read);

            if (File.Exists(strFilename))
                XmlTextReader rdrXml = new XmlTextReader(fsVideos );
            else
                Console.WriteLine("The file {0} could not be located",
                                  strFilename);

            fsVideos.Close();
            Console.WriteLine();
            return 0;
        }
    }
}

To actually read the file, the XmlTextReader is equipped with the Read() method whose syntax is:

public override bool Read();

As you may suspect, this method only tells you that it successfully read an item. It doesn't tell you what it read. As stated already, the XmlTextReader scans a file in a left-right-top-down approach. When it has read something, it returns true. If it didn't or couldn't read something, it returns false. Therefore, you can call it to read an item. If it succeeds, it returns true. After reading that item, you can call it again to move to the next item. If there is a next item, it reads it and returns true. But, if there is no next item, the Read() method would not be able to read it and it would return false. In other words, you can ask the Read() method to continuously read the items as long as it returns true. Once it cannot read an item, you can ask it to stop. To perform this exercise, you can use either a while or a do...while loop. This would be done as follows:

using System;
using System.IO;
using System.Xml;

namespace VideoCollection1
{
    class Program
    {
        static int Main(string[] args)
        {
            string strFilename = "Videos.xml";
            FileStream fsVideos = new FileStream(strFilename,
                                                 FileMode.Open,
                                                 FileAccess.Read);

            XmlTextReader rdrXml = new XmlTextReader(fsVideos );

            do {
                // Read an item and return true
                // Continue reading as long as ...
            } while (rdrXml.Read() == true); // ... as long as Read() returns true
	    // Once Read() returns false, STOP!!!
            

            fsVideos.Close();
            Console.WriteLine();
            return 0;
        }
    }
}

To identify what was read, the XmlTextReader provides methods appropriate for the different types of item that an XML file can contain. Starting in the next lesson, we will review the types of items of a file.

Earlier, we mentioned that XML worked through markups. A simple markup is made of a tag created between the left angle bracket "<" and the right angle bracket ">". Just creating a markup is not particularly significant. You must give it meaning. To do this, you can type a number, a date, or a string on the right side of the right angle bracket ">" symbol. The text on the right side of ">" is referred to as the item's text. It is also called a value.

After specifying the value of the markup, you must close it: this is a rule not enforced in HTML but must be respected in XML to make it "well-formed". To close a tag, use the same formula of creating a tag with the left angle bracket "<", the tag, and the right angle bracket ">" except that, between < and the tag, you must type a forward slash. The formula to use is:

<tag>some value</tag>

The item on the left side of the "some value" string, in this case <tag>, is called the opening or start-tag. The item on the right side of the "some value" string, in this case </tag>, is called the closing or end-tag. Like<tag> is a markup, </tag> also is called a markup.

With XML, you create your own tags with custom names. This means that a typical XML file is made of various items. Here is an example: