Introduction to XML Attributes

When studying XML elements we saw how they constitute the main objects of an XML document. We also saw that an element could 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

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 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.

1. Start Notepad and type the following:
    

   <?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>

2. To save the file, on the main menu, click File -> New
3. Locate your VBasic folder and display it in the Save In combo box
4. Click the Create New Folder button and type Countries1
5. Display the new Countries1 folder in the Save In combo box
6. Change the Files of Type to All Files
7. Change the file name to Countries.xml
8. Click Save
9. In the empty document, type the following:
    

   Imports System Imports System.xml Module Exercise     Public Sub Main()         End Sub End Module

10. Save the file as Exercise.vb in your Countries1 folder

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:

Imports System
Imports System.xml

Module Exercise

    Private Sub CreateAttribute()
        ' Open the XML file
        Dim docXML As XmlDocument = 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

    Public Sub Main()
        CreateAttribute()
    End Sub

End Module

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, this method produces an XmlAttribute object. To add the new attribute to an element, you can call the XmlElement.SetAttributeNode() 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. Here is an example that looks for a particular video in a collection and adds an ISBN attribute to it:

Practical Learning: Creating an Attribute

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.

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.RemoveAt() 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.

Here is an example:

Imports System
Imports System.xml

Module Exercise
    Private Sub CreateContinent()
        ' Open the XML file
        Dim xmlDocContinents As XmlDocument = New XmlDocument
        xmlDocContinents.Load("Countries.xml")

        Dim strContinent As String = Nothing

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

        ' 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
        xmlNewContinent.SetAttribute("Name", strContinent)

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

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

    Private Sub RemoveContinents()
        ' Open the XML file
        Dim xmlDocContinents As XmlDocument = 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

    Public Sub Main()
        RemoveContinents()
    End Sub

End Module