XML.NET and C# - Lesson 8: The Attributes of an XML Element

	The Attributes of an XML Element

Introduction

So far, we have used only one attribute per element. Fortunately, you can create as many attributes as you judge necessary in an element. To do this, type the name of each attribute, assign it a double-quoted element and separate the attribute from the next with an empty space. Here is an example of an element with different attributes:

<Video ISBN="0-7888-1623-3" ScreenRatio="Standard" SoundtrackAvailable="True" />

As mentioned already and as you must always remember, attributes belong to an element. To support them, the attributes of an element are stored in the XmlElement.Attributes property and held by a class called XmlAttributeCollection. The XmlAttributeCollection class is based on the XmlNamedNodeMap class. This class lays a foundation to access attributes using their names or index in the collection. To know the number of attributes in an element, you can use the XmlNamedNodeMap.Count property.

Practical Learning: Adding Attributes

Start Notepad and type the following:

<?xml version="1.0" encoding="utf-8" ?>
<World Area="510,072,000,000" Population="6,379,157,361">
    <Continent Name="Africa" Area="30,065,000" Population="807,419,000">
        <Country CountryName="Burundi" Area="27,830" Population="6,231,221" Capital="Bujumbura" Code="bi" />
    </Continent>
    <Continent Name="Europe" Area="9,938,000" Population="730,916,000">
        <Country CountryName="Italy" Area="301,230" Population="58,057,477" Capital="Rome" Code="it" />
    </Continent>
</World>

To save the file, on the main menu, click File -> New
Locate your CSharp Lessons folder and display it in the Save In combo box
Click the Create New Folder button and type Countries2
Display the new Countries1 folder in the Save In combo box
Change the Files of Type to All Files
Change the file name to Countries.xml
Click Save

In the empty document, type the following:

// Project Name: Countries
// Purpose:      To list countries statistics

using System;
using System.Xml;

namespace CountriesStatistics
{
    class Exercise
    {
        static int Main()
        {
	    return 0;
        }
    }
}

Save the file as Exercise in your Countries2 folder

Access to an Attribute

To access an attribute by its position in the collection, you can use the XmlNamedNodeMap.Item() method.

The XmlAttributeCollection class is equipped with an ItemOf indexed property. This property is overloaded in three versions. The first version has the following syntax:

public virtual XmlAttribute this[int i] {get;}

This property allows you to access an attribute by considering that the attributes are stored in an array. The first or most left attribute has an index of 0; the second attribute from left (of course without counting the name of the element) has an index of 1, and so on.

It can be difficult and sometimes unpredictable in some scenarios if you try accessing an attribute by their indices because you must know exactly where each attribute is positioned. Consider the following version of our Videos.xml XML file:

<?xml version="1.0" encoding="utf-8" ?>
<Videos FileDesc="Personal Video Collection">
    <Video ISBN="0-7888-1623-3" ScreenRatio="Standard" SoundtrackAvailable="True">
        <Title StoryBy="Marty Kaplan and Jonathan Reynold" Screenplay="Marty Kaplan">The Distinguished Gentleman</Title>
        <Director>Jonathan Lynn</Director>
        <Actors></Actors>
        <Length>112 Minutes</Length>
        <Format>DVD</Format>
        <Rating>R</Rating>
    </Video>
    <Video ISBN="0-7907-3900-3">
        <Title Screenplay="Charlie Peter">Her Alibi</Title>
        <Director>Bruce Beresford</Director>
        <Length>94 Mins</Length>
        <Format>DVD</Format>
        <Rating>PG-13</Rating>
    </Video>
</Videos>

In the first video, the name of the screenplay writer is stored at index 1. In the second video, the name of the screenplay writer is stored at index 0. In this case, it may not be a good item to use the index to locate an attribute. Fortunately, the second version of the overloaded XmlAttributeCollection.ItemOf[] property has the following syntax:

public virtual XmlAttribute this[string name] {get;}

With this version, you can explicitly specify the name of the attribute that you want.

Practical Learning: Accessing an Attribute

To display the continents, change the code as follows:

// Project Name: Countries
// Purpose:      To list countries statistics

using System;
using System.Xml;

namespace CountriesStatistics
{
        class Exercise
        {
	    private static void ShowContinents()
	    {
		// Open the XML file
		XmlDocument xmlDocContinents = new XmlDocument();
		xmlDocContinents.Load("Countries.xml");

		// Get a list of elements whose names are Continent
		XmlNodeList lstContinents =
			xmlDocContinents.GetElementsByTagName("Continent");

		// Show the statistics on the continents
		Console.WriteLine(" =-= Continents =-=");
		Console.WriteLine("Name\tArea\t\tPopulation");
		for(int i = 0; i < lstContinents.Count; i++)
		{
		    Console.WriteLine("{0}\t{1}\t{2}",
		                  lstContinents[i].Attributes["Name"].InnerText,
		                  lstContinents[i].Attributes["Area"].InnerText,
		                  lstContinents[i].Attributes["Population"].InnerText);
		}
	    }

       	    static int Main()
       	    {
		ShowContinents();

		return 0;
        	    }
        }
}

Save the file
Open the Command Prompt and Change to the Directory that contains the above file, Countries2
To compile the exercise, type csc Exercise.cs and press Enter

To execute the application, type Exercise and press Enter

=-= Continents =-=
Name    Area            Population
Africa  30,065,000      807,419,000
Europe  9,938,000       730,916,000

Return to Notepad

Attribute Addition

Whether using its index or name, after accessing an attribute, you can manipulate it as you see fit. For example, you can change or delete it using the same techniques we saw to perform on an individual attribute.

As mentioned already, the attributes are stored as a list. Because you have complete access to this list and the positions of its attributes, when creating or adding a new attribute, you can specify the position the new attribute should have in the collection. To create an attribute as the first in an element, you can call the XmlAttributeCollection.Prepend() method. Its syntax is:

public virtual XmlAttribute Prepend(XmlAttribute node);

Another technique you can use consists of locating an attribute first. Once you have one, to create a new attribute before it, you can call the XmlAttributeCollection.InsertBefore() method. Its syntax is:

public virtual XmlAttribute InsertBefore(XmlAttribute newNode, XmlAttribute refNode);

To add a new attribute after the current one, you can call the XmlAttributeCollection.InsertAfter() method. Its syntax is:

public virtual XmlAttribute InsertAfter(XmlAttribute newNode, XmlAttribute refNode);

To add an attribute at the end of the list of attributes of an element, you can call the XmlAttributeCollection.Append() method. Its syntax is:

public virtual XmlAttribute Append(XmlAttribute node);

Practical Learning: Creating Attributes

To allow the user to create a new continent, change the program as follows:

// Project Name: Countries
// Purpose:      To list countries statistics

using System;
using System.Xml;

namespace CountriesStatistics
{
        class Exercise
        {
	    private static void ShowContinents()
	    {
		// Open the XML file
		XmlDocument xmlDocContinents = new XmlDocument();
		xmlDocContinents.Load("Countries.xml");

		// Get a list of elements whose names are Continent
		XmlNodeList lstContinents =
			xmlDocContinents.GetElementsByTagName("Continent");

		// Show the statistics on the continents
		Console.WriteLine(" =-= Continents =-=");
		Console.WriteLine("Name\tArea\t\tPopulation");
		for(int i = 0; i < lstContinents.Count; i++)
		{
		    Console.WriteLine("{0}\t{1}\t{2}",
		                  lstContinents[i].Attributes["Name"].InnerText,
		                  lstContinents[i].Attributes["Area"].InnerText,
		                  lstContinents[i].Attributes["Population"].InnerText);
		}
	}

	private static void CreateNewContinent()
	{
	    string strContinent  = null;
	    string strArea       = null;
	    string strPopulation = null;

	    // Open the XML file
	    XmlDocument xmlDocContinents = new XmlDocument();
	    xmlDocContinents.Load("Countries.xml");

	    // Create a Continent element that the new attribute will be added to
	    XmlElement xmlNewContinent = xmlDocContinents.CreateElement("Continent");

	    // Present the current list of continents to the user
	    ShowContinents();

	    // Request the name of a continent from the user
	    Console.Write("Enter a new continent: ");
	    strContinent = Console.ReadLine();

	    // Create a Name attribute using the continent that the user entered
	    xmlNewContinent.SetAttribute("Name", strContinent);

	    // Request the continent's area from the user
	    Console.Write("Enter the area of the continent: ");
	    strArea = Console.ReadLine();

	    // Create the Area attribute
	    xmlNewContinent.SetAttribute("Area", strArea);

	    // Request the population of the continent from the user
	    Console.Write("Enter the population of the continent: ");
	    strPopulation = Console.ReadLine();

	    // Create the Population attribute
	    xmlNewContinent.SetAttribute("Population", strPopulation);

	    // Add the element and its attribute to the document
	    xmlDocContinents.DocumentElement.AppendChild(xmlNewContinent);

	    // Save the XML file
	    xmlDocContinents.Save("Countries.xml");
	}

       	static int Main()
       	{
	    int choice = 0;

	    Console.WriteLine(" =-= Main Menu =-=");
	    Console.WriteLine("0 - Quit");
	    Console.WriteLine("1 - Display Continents");
	    Console.WriteLine("2 - Create New Continent");
	    Console.Write("Your Choice? ");
	    choice = int.Parse(Console.ReadLine());

	    switch(choice)
	    {
	    case 1:
		ShowContinents();
		break;
	    case 2:
		CreateNewContinent();
		break;
	    }

	    return 0;
        }
    }
}

Save the file and switch to the Command Prompt
Compile and execute the application to test it
Return to Notepad

To allow the user to create a new country, change the file as follows:

// Project Name: Countries
// Purpose:      To list countries statistics

using System;
using System.Xml;

namespace CountriesStatistics
{
        class Exercise
        {
	    private static void ShowContinents()
	    {
		// Open the XML file
		XmlDocument xmlDocContinents = new XmlDocument();
		xmlDocContinents.Load("Countries.xml");

		// Get a list of elements whose names are Continent
		XmlNodeList lstContinents =
			xmlDocContinents.GetElementsByTagName("Continent");

		// Show the statistics on the continents
		Console.WriteLine(" =-= Continents =-=");
		Console.WriteLine("Name\tArea\t\tPopulation");
		for(int i = 0; i < lstContinents.Count; i++)
		{
		    Console.WriteLine("{0}\t{1}\t{2}",
		                  lstContinents[i].Attributes["Name"].InnerText,
		                  lstContinents[i].Attributes["Area"].InnerText,
		                  lstContinents[i].Attributes["Population"].InnerText);
		}
	}

	private static void CreateNewContinent()
	{
	    string strContinent  = null;
	    string strArea       = null;
	    string strPopulation = null;

	    // Open the XML file
	    XmlDocument xmlDocContinents = new XmlDocument();
	    xmlDocContinents.Load("Countries.xml");

	    // Create a Continent element that the new attribute will be added to
	    XmlElement xmlNewContinent = xmlDocContinents.CreateElement("Continent");

	    // Present the current list of continents to the user
	    ShowContinents();

	    // Request the name of a continent from the user
	    Console.Write("Enter a new continent: ");
	    strContinent = Console.ReadLine();

	    // Create a Name attribute using the continent that the user entered
	    xmlNewContinent.SetAttribute("Name", strContinent);

	    // Request the continent's area from the user
	    Console.Write("Enter the area of the continent: ");
	    strArea = Console.ReadLine();

	    // Create the Area attribute
	    xmlNewContinent.SetAttribute("Area", strArea);

	    // Request the population of the continent from the user
	    Console.Write("Enter the population of the continent: ");
	    strPopulation = Console.ReadLine();

	    // Create the Population attribute
	    xmlNewContinent.SetAttribute("Population", strPopulation);

	    // Add the element and its attribute to the document
	    xmlDocContinents.DocumentElement.AppendChild(xmlNewContinent);

	    // Save the XML file
	    xmlDocContinents.Save("Countries.xml");
	}

	private static void AddCountry()
	{
	    string strContinent  = null;
	    string strCountry    = null;
	    string strArea       = null;
	    string strPopulation = null;
	    string strCapital    = null;
	    string strCode       = null;

	    // Open the XML file
	    XmlDocument xmlDocContinents = new XmlDocument();
	    xmlDocContinents.Load("Countries.xml");
	 
	    // Display the list of continents to the user
	    Console.WriteLine("Here is a list of the created continents");
	    ShowContinents();

	    // Request a continent from the user
	    Console.Write("Enter the desired continent: ");
	    strContinent = Console.ReadLine();

	    // Get a list of elements whose names are Continent
	    XmlNodeList lstContinents = xmlDocContinents.GetElementsByTagName("Continent");

	    // Visit each Continent element
	    for(int i = 0; i < lstContinents.Count; i++)
	    {
		// Get a list of the attributes of the current element
		XmlAttributeCollection curAttributes = lstContinents[i].Attributes;

		// Check each attribute, looking for the continent that the user entered
		for(int j = 0; j < curAttributes.Count; j++)
		{
		    // Check if the current continent is the same that the user selected
		    if( curAttributes["Name"].InnerText == strContinent )
		    {
			// Once you find one, get its XmlElement reference
			XmlElement elmNewCountry = xmlDocContinents.CreateElement("Country");

			// Request the name of a country from the user
			Console.Write("Enter the name of the country: ");
			strCountry = Console.ReadLine();
			// Create the country specified by the user
			elmNewCountry.SetAttribute("CountryName", strCountry);

			// Request the area of the country from the user
			Console.Write("Enter the area of the country: ");
			strArea = Console.ReadLine();
			// Create the Area specified by the user
			elmNewCountry.SetAttribute("Area", strArea);

			// Request the population of the country from the user
			Console.Write("Enter the area of the country: ");
			strPopulation = Console.ReadLine();
			// Create the Population attribute
			elmNewCountry.SetAttribute("Population", strPopulation);

			// Request the Capital of the country from the user
			Console.Write("Enter the area of the country: ");
			strCapital = Console.ReadLine();
			// Create the Capital attribute
			elmNewCountry.SetAttribute("Capital", strCapital);

			// Request the Internet Code of the country from the user
			Console.Write("Enter the population of the country: ");
			strCode = Console.ReadLine();
			// Create the Internet Code attribute
			elmNewCountry.SetAttribute("Code", strCode);

			// Add the element (and its attribute) as a child of the current Continent
			lstContinents[i].AppendChild(elmNewCountry);
				 
			// Save the XML file
			xmlDocContinents.Save("Countries.xml");

			break;
		    }
		}
	    }
	}

       	static int Main()
       	{
	    int choice = 0;

	    Console.WriteLine(" =-= Main Menu =-=");
	    Console.WriteLine("1 - Display Continents");
	    Console.WriteLine("2 - Create New Continent");
	    Console.WriteLine("3 - Create New Country");
	    Console.Write("Your Choice? ");
	    choice = int.Parse(Console.ReadLine());

	    switch(choice)
	    {
	    case 1:
		ShowContinents();
		break;
	    case 2:
		CreateNewContinent();
		break;
	    case 3:
		AddCountry();
		break;
	    }

	    return 0;
        }
    }
}

Save the file and switch to the Command Prompt
Compile and execute the application to test it
Create some continents and add some countries to them
Type Exit and press Enter to close the Command Prompt
Close Notepad to end the lesson

The Parent of an Attribute

Once an attribute has been created, to identify the element it belongs to, you can access its XmlAttribute.OwnerElement property. This property produces an XmlElement value.

Attribute Removal

If you find out that an element has one or more undesired attributes, you can delete them and you have various options to perform this operation.

Since the attributes are stored in a collection, you can locate the undesired attribute by its index and then delete it. To do this, you can call the XmlAttributeCollection.RemoveAt() method. Its syntax is:

public virtual XmlAttribute RemoveAt(int i);

This method expects the index of the attribute that needs to be removed. As mentioned for the XmlAttributeCollection.ItemOf indexed property, to efficiently use this RemoveAt() method, you should know the exact index of the attribute, otherwise, you may access and therefore delete the wrong attribute. An alternative is to explicitly identify the attribute you want to delete. To do this, you can call the XmlAttributeCollection.Remove() method. Its syntax is:

public virtual XmlAttribute Remove(XmlAttribute node);

This method takes as attribute the XmlAttribute identification of the attribute you want to remove.

To delete all attributes of an element, you can call the XmlAttributeCollection.RemoveAll() method. Its syntax is:

public virtual void RemoveAll();

This method would simply remove all attributes that belong to an XmlElement object.


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