Visual Basic .NET XML - 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

To manually create some attributes, change the Countries.xml file as follows:

<?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>

Change the design of the New Country form as follows:

Country	Name	Text	Other Properties
Label		Continent:
ComboBox	cboContinents		DropDownStyle: DropDownList
Button	btnNewContinent	New Continent
TextBox	txtNewCountry
Label		Area (sq km):
TextBox	txtArea		TextAlign: Right
Label		Capital:
TextBox	txtCapital
Label		Population:
TextBox	txtPopulation		TextAlign: Right
Label		Internet Code:
TextBox	txtInternetCode		TextAlign: Right
Button	btnAddCountry	Add Country
Button	btnClose	Close
Form			AcceptButton: btnAddCountry MaximizeBox: False StartPosition: CenterScreen

Change the design of the New Continent dialog box as follows:

Control	Name	Text	Other Properties
Label		Continent Name:
TextBox	txtContinentName		Modifiers: Public
Label		Area:
TextBox	txtArea		Modifiers: Public TextAlign: Right
Label		Population:
TextBox	txtPopulation		Modifiers: Public TextAlign: Right

Save all

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];

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:

Overloads Public Overridable Default ReadOnly Property ItemOf(ByVal name As String) As XmlAttribute

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

Practical Learning: Accessing an Attribute

Display the New Country form.
Double-click an unoccupied area of the form to generate its Load event and implement it as follows:

Private Sub NewCountry_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ' Open the XML file
        Dim xmlDocContinents As New XmlDocument
        xmlDocContinents.Load("Countries.xml")

        ' Get a list of elements whose names are Continent
        Dim lstContinents As XmlNodeList = xmlDocContinents.GetElementsByTagName("Continent")

        ' Retrieve the name of each continent and put it in the combo box
        Dim i As Integer
        For i = 0 To lstContinents.Count Step 1
            cboContinents.Items.Add(lstContinents(i).Attributes("Name").InnerText)
        Next
End Sub

Execute the application and display the New Country dialog box to make sure its combo box is filled with available continent names
Close the forms and return to your programming environment

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 reviewed for 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 Overridable Function Prepend(ByVal node As XmlAttribute) As XmlAttribute

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 Overridable Function InsertBefore(ByVal newNode As XmlAttribute, _
                                                                 ByVal refNode As XmlAttribute) As XmlAttribute

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

Public Overridable Function InsertAfter(ByVal newNode As XmlAttribute, _
                                                              ByVal refNode As XmlAttribute) As XmlAttribute

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 Overridable Function Append(ByVal node As XmlAttribute) As XmlAttribute

Practical Learning: Creating Attributes

For the New Country form, change the Click event of the btnNewContinent button as follows:

Private Sub NewCountry_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ' Open the XML file
        Dim xmlDocContinents As New XmlDocument
        xmlDocContinents.Load("Countries.xml")

        ' Get a list of elements whose names are Continent
        Dim lstContinents As XmlNodeList = xmlDocContinents.GetElementsByTagName("Continent")

        ' Retrieve the name of each continent and put it in the combo box
        Dim i As Integer
        For i = 0 To lstContinents.Count - 1
            cboContinents.Items.Add(lstContinents(i).Attributes("Name").InnerText)
        Next i
End Sub

Return to the New Country form.
Double-click the Add Country button and implement its Click event as follows:

Private Sub btnAddCountry_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAddCountry.Click
        ' If the user didn't select a continent, don't do anything
        If cboContinents.SelectedIndex = -1 Then
            Exit Sub
        End If

        ' Open the XML file
        Dim xmlDocContinents As New XmlDocument
        xmlDocContinents.Load("Countries.xml")

        ' The continent selected by the user
        Dim strContinent As String = cboContinents.Text
        ' Get a list of elements whose names are Continent
        Dim lstContinents As XmlNodeList = xmlDocContinents.GetElementsByTagName("Continent")

        ' Visit each Continent element
        Dim i As Integer
        Dim j As Integer
        For i = 0 To lstContinents.Count - 1
            ' Get a list of the attributes of the current element
            Dim curAttributes As XmlAttributeCollection = lstContinents(i).Attributes

            ' Check each attribute, looking for the continent that the user selected
            For j = 0 To curAttributes.Count - 1
                ' Check if the current continent is the same that the user selected
                If curAttributes("Name").InnerText = strContinent Then
                    ' Once you find one, get its XmlElement reference
                    Dim elmNewCountry As XmlElement = xmlDocContinents.CreateElement("Country")

                    ' Create each attribute only if the user specified it

                    ' Create the Name using the New Country specified by the user
                    If txtNewCountry.Text <> "" Then
                        elmNewCountry.SetAttribute("CountryName", txtNewCountry.Text)
                    End If

                    ' Create the Area attribute
                    If txtArea.Text <> "" Then
                        elmNewCountry.SetAttribute("Area", txtArea.Text)
                    End If

                    ' Create the Population attribute
                    If txtPopulation.Text <> "" Then
                        elmNewCountry.SetAttribute("Population", txtPopulation.Text)
                    End If

                    ' Create the Capital attribute
                    If txtCapital.Text <> "" Then
                        elmNewCountry.SetAttribute("Capital", txtCapital.Text)
                    End If

                    ' Create the Code attribute
                    If txtInternetCode.Text <> "" Then
                        elmNewCountry.SetAttribute("Code", txtInternetCode.Text)
                    End If

                    ' 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")
                End If
            Next j
        Next i

        txtNewCountry.Text = ""
        txtArea.Text = ""
        txtPopulation.Text = ""
        txtCapital.Text = ""
        txtInternetCode.Text = ""
        txtNewCountry.Focus()
End Sub

Double-click the Close button and implement its Click event as follows:

Private Sub btnClose_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnClose.Click
        Close()
End Sub

Execute the application to test it
Create some continents and add some countries to them

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 Overridable Function RemoveAt(ByVal i As Integer) As XmlAttribute

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 Overridable Function Remove(ByVal node As XmlAttribute) As XmlAttribute

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 Overridable Sub RemoveAll()

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