Visual Basic .NET XML - Lesson 7: Introduction to XML Attributes

	Introduction to XML Attributes

Fundamentals of Attributes

Introduction

When studying XML elements we saw how they constitute the main objects of an XML document. We also saw that an element can be nested inside of another element. Instead of nesting an element, you can transform the nested element into being part of the nesting element and thereby giving away its element qualities. This is the basis of an attribute.

An attribute is a value that is created as part of an element, making that value different from the value of a regular element. There are similarities and differences between an element and an attribute.

The element and the attribute have these in common:

Both (must) have a name
Each may or may not have a value

The differences between an element and an attribute are:

An attribute is considered a characteristic of an element. This means that an attribute belongs to an element
An element can have one or more attributes. As mentioned already, an attribute cannot have an element
An attribute must be created in the start-tag of an element
An element cannot be defined as part of an attribute

Practical Learning: Introducing Attributes

Start a new Windows Application named Countries1
To add an XML file, on the main menu, click File -> New -> File...
In the Templates list of the New File dialog box, click XML File (.xml) and click Open
To save the file, on the the Standard toolbar, click the Save button
Locate the folder of the current project and double-click its bin sub-folder to display it in the Save In combo box
Change the file name to Countries and click Save

The Name and Value of an Attribute

An attribute must be created inside the start-tag of an element. To manually create an attribute, type the left angle bracket of the element, followed by the name of the element, an empty space, the name of the attribute, and assign it a value as a string. Imagine you have an ISBN element as a child of a Video element as follows:

<Video>
	<ISBN>0-7888-1623-3</ISBN>
</Video>

In this case, since ISBN is simply a child of the Video element, you can change the ISBN element to become an attribute of the Video element as follows:

<Video ISBN="0-7888-1623-3">

Now, ISBN is an attribute of the Video element.

An attribute mostly gets meaning because of its text. This text allows you to know what the attribute is holding at one particular time. One of three properties can be used to specify or to retrieve the text held by an attribute. The XmlAttribute.Value is the most commonly used property to get or set this information. Besides Value, you can also use XmlAttribute.InnerText or XmlAttribute.InnerXml to access the text of an attribute.

An Element With a Single Attribute

Introduction

An element can have 0, one, or more attributes. 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.

Before performing an attribute-related operation on an element, to find out whether the element has any attribute, you can check the value of the Boolean XmlElement.HasAttributes property. If this property produces a true value, then the element has at least one attribute; otherwise, the element doesn't have any.

While a certain element may have an attribute, a sibling element with the same name may not have an attribute or may have a completely different type of attribute. Here is an XML file with attributes in some elements:

<?xml version="1.0" encoding="utf-8" ?>
<Videos>
	<Video ISBN="0-7888-1623-3">
		<Title 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>
		<Title WrittenBy="Charlie Peter">Her Alibi</Title>
		<Director>Bruce Beresford</Director>
		<Length>94 Mins</Length>
		<Format>DVD</Format>
		<Rating>PG-13</Rating>
	</Video>
</Videos>

Remember that you can include white spaces to make your code easy to read. This means that you can type an attribute on the next line of its element name. In the previous lesson, we saw that every element must be closed. We saw that you can close an element with an end-tag as follows:

<Video><ISBN>0-7888-1623-3</ISBN></Video>

We also saw that you can close an element locally as follows: <Video />. If you create an attribute in an empty element, you can also close it by typing the indicative forward slash before the right angle bracket and after an empty space. Here is an example:

<Video ISBN="0-7888-1623-3" />

To support attributes, the System.Xml namespace provides the XmlAttribute class which is derived from XmlNode. One of the two most important properties of an attribute is its name, represented by Name. Every attribute must have a name, which is used to identify the attribute. For one thing, you must be able to locate an attribute in an element, for another, if an element has many attributes, you need to be able to identify one of them by name.

Practical Learning: Creating Simple Attributes

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

<?xml version="1.0" encoding="utf-8" ?>
<World>
	<Continent Name="Africa"></Continent>
	<Continent Name="Europe"></Continent>
	<Continent Name="Asia"></Continent>
	<Continent Name="South America"></Continent>
</World>

Save all

Attribute Creation

As mentioned already, an attribute primarily belongs to an element. This means that, when creating an attribute, you must specify what element it would belong to. To support the attributes of an element, the XmlElement class is equipped with the SetAttribute() method which is overloaded in two versions. The first version of this method has the following syntax:

Overloads Public Overridable Sub SetAttribute(ByVal name As String, ByVal value As String)

The first argument is the name of the new attribute and the second argument will be its text. Before adding an attribute, you should first identify its parent element. Here is an example that adds an attribute to the root element:

Private Sub btnNewAttribute_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewAttribute.Click
        ' Open the XML file
        Dim docXML As New XmlDocument
        docXML.Load("Videos.xml")

        ' Create an attribute and add it to the root element
        docXML.DocumentElement.SetAttribute("FileDesc", "Personal Video Collection")
        docXML.Save("Videos.xml")
End Sub

From the above Videos.xml file, this code would result in:

<?xml version="1.0" encoding="utf-8"?>
<Videos FileDesc="Personal Video Collection">
  <Video ISBN="0-7888-1623-3">
    <Title 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>
    <Title WrittenBy="Charlie Peter">Her Alibi</Title>
    <Director>Bruce Beresford</Director>
    <Length>94 Mins</Length>
    <Format>DVD</Format>
    <Rating>PG-13</Rating>
  </Video>
</Videos>

To support attribute addition, the XmlDocument class is equipped with the CreateAttribute() method, which is overloaded in three versions. The first version of this method has the following syntax:

Overloads Public Function CreateAttribute(ByVal name As String) As XmlAttribute

This method expects the name of the attribute as argument. If it succeeds, it produces an XmlAttribute object. To add the new attribute to an element, you can call the XmlElement.SetAttributeNote() method. This method is overloaded in two versions. One of the versions (the first) uses the following syntax:

Overloads Public Overridable Function SetAttributeNode(ByVal newAttr As XmlAttribute) As XmlAttribute

This method expects an XmlAttribute as a constant pointer.

Practical Learning: Creating an Attribute

To add a new form, on the main menu, click Project -> Add Windows Form...
In the Templates list of the Add New Item dialog box, make sure Windows Form is selected
Set the Name to NewContinent and click Open

Design the form as follows:

Control	Name	Text	Other Properties
Label		Continent Name:
TextBox	txtContinentName		Modifiers: Public
Button	btnOK	OK	DialogResult: OK
Button	btnCancel	Cancel	DialogResult: Cancel
Form		New Continent	AcceptButton: btnOK CancelButton: btnCancel FormBorderStyle: FixedDialog MaximizeBox: False MinimizeBox: False ShowInTaskbar: False StartPosition: CenterScreen

To add a new form, on the main menu, click Project -> Add Windows Form...
In the Templates list of the Add New Item dialog box, make sure Windows Form is selected
Set the Name to NewCountry and click Open
Add a new Button to the form and set its Properties as follows:
Name: btnNewContinent
Text: New Continent
Double-click the New Continent button
In the top section of the file, above the Public Class NewCountry line, type

Imports System.Xml

Implement its Click event as follows:

Private Sub btnNewContinent_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewContinent.Click
        Dim frmContinent As New NewContinent

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

        ' Display the New Continent dialog box and find out if the user clicked OK
        If frmContinent.ShowDialog() = DialogResult.OK Then
            If frmContinent.txtContinentName.Text = "" Then
                Exit Sub
            End If

            ' Create an element that the new attribute will be added to
            Dim xmlNewContinent As XmlElement = xmlDocContinents.CreateElement("Continent")

            ' Create a Continent element and set its value to
            ' that of the New Continent dialog box
            xmlNewContinent.SetAttribute("Name", frmContinent.txtContinentName.Text)

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

            ' Save the XML file
            xmlDocContinents.Save("Countries.xml")
        End If
End Sub

Display the first or main form (Form1.vb [Design]) and add a new Button control to it
Set its Name to btnNewCountry and its Text to New Country

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

Private Sub btnNewContinent_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnNewContinent.Click
        Dim frmContinent As New NewContinent

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

        ' Display the New Continent dialog box and find out if the user clicked OK
        If frmContinent.ShowDialog() = DialogResult.OK Then
            If frmContinent.txtContinentName.Text = "" Then
                Exit Sub
            End If

            ' Create an element that the new attribute will be added to
            Dim xmlNewContinent As XmlElement = xmlDocContinents.CreateElement("Continent")

            ' Create a Continent element and set its value to
            ' that of the New Continent dialog box
            xmlNewContinent.SetAttribute("Name", frmContinent.txtContinentName.Text)

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

            ' Save the XML file
            xmlDocContinents.Save("Countries.xml")
        End If
End Sub

Execute the application and test it by creating a new continent whose name is North America
Close the form and return to your programming environment

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 an element has an attribute you don't want or that you don't need anymore, you can delete that attribute. You have various options, two are available through the XmlElement class.

The attributes of an XmlElement are considered stored in an indexed list (in the next sections, we will see that the attributes are stored in a collection) with the most left attribute at index 0, the second from left at index 1, and so on. Based on this, to remove an attribute by locating it based on its index, you can call the XmlElement.RemoveAttributeAt() method. Its syntax is:

Public Overridable Function RemoveAttributeAt(ByVal i As Integer) As XmlNode

When calling this method, if an attribute exists at position i, it will be deleted and the method would return it. If there is no attribute at that index, the method doesn't do anything and it returns 0.

Using the XmlElement.RemoveAttributeAt() method to delete an attribute can be uncertain because you would not know whether there is an attribute at the specified position. An alternative is to specify the name of the attribute you want to delete. To support this, the XmlElement class is equipped with the RemoveAttribute() method, which is overloaded with two versions. One of the versions (the first) of this method uses the following syntax:

Overloads Public Overridable Sub RemoveAttribute(ByVal name As String)

This method accepts as argument the name of the attribute to remove.

Another technique you can use consists of defining an XmlAttribute object and submitting to its XmlElement parent to delete. To do this, you can use the XmlElement.RemoveAttributeNode() method. Its syntax is:

Overloads Public Overridable Function RemoveAttributeNode(ByVal oldAttr As XmlAttribute) As XmlAttribute

When calling this method, pass the attribute object as argument. If the attribute exists, it would be removed and the method would return the deleted attribute. If the attribute doesn't exist, nothing would happen.

Practical Learning: Removing Attributes

Access the NewCountry.vb file and change the code of the btnNewContinent Click event as follows:

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

        ' Since we are planning to create new elements and their attributes
        ' delete all continents
        Dim elmRoot As XmlElement = xmlDocContinents.DocumentElement
        elmRoot.RemoveAll()

        ' Save the XML file
        xmlDocContinents.Save("Countries.xml")
End Sub

Execute the application to test it
Display the New Country form and click its New Continent button
Close the forms and return to your programming environment


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