Advanced XML Data Reading with VB.NET

Introduction

Topics regarding XML prove increasingly popular. It may be because XML is one of the latest “hot terms“; or it may be due to the complexity revolving around XML technologies. With this article I will show you three different ways to read and use XML documents productively in your VB.NET applications.

XML Parsing

There are essentially three main ways to read XML files. These are:

  1. SAX. Simple API (Application Programming Interface) for XML
  2. DOM API. Document Object Model Application Programming Interface
  3. XPath

SAX

SAX comes in very handy, especially when you have to read large XML files. With SAX the entire file is never in memory at the same time. It lets the built in parser decide which area of the file will be read and which will be placed in memory. Simple API for XML is a nice article on SAX.

DOM

There’s not much difference between the DOM API and the SAX API, except for the fact that with DOM, the whole file is read into memory, irrespective of the file’s size. So the limitation here is to not have great flexibility with reading large XML files. DOM and SAX is a slideshow about the DOM API.

XPath

XPath comes in very handy when reading XML files that are online. Wikipedia has more details on XPath.

Our Project

Create a new VB.NET Windows Forms application. You can name it anything you like. Design it to resemble Figure 1 below.

Design
Figure 1Design

Before we start with the VB.NET project, let’s create an XML file to be used with this example. Open Notepad and edit it as follows:

<?xml version="1.0"?>

<BLURays>
	<Movie Title="Fallen">
   		<Actor>Denzel Washington</Actor>
   		<Type>Thriller</Type>
	</Movie>
	<Movie Title="Mad Max">
   		<Actor>Mel Gibson</Actor>
   		<Type>Sci-Fi</Type>
	</Movie>
	<Movie Title="Event Horizon">
   		<Actor>Sam Neill</Actor>
   		<Type>Sci-Fi Horror</Type>
	</Movie>
	<Movie Title="Happy Gilmore">
   		<Actor>Adam Sandler</Actor>
   		<Type>Comedy</Type>
	</Movie>
	<Movie Title="The Postman">
   		<Actor>Kevin Costner</Actor>
   		<Type>Adventure</Type>
	</Movie>
</BLURays>

This is just a simple XML file containing some of my favorite movies and their respective actors and genre. You will be using this file throughout today’s example.

Code

In order to read any XML file and manipulate its data, we need certain namespaces. Add them now:

Imports System.Xml
Imports System.Xml.XPath

The first namespace will help you manipulate XML files with either the SAX API or DOM API. The second namespace is used specifically with the XPath logic.

Reading XML Files with the SAX API

Add the following code behind the button labelled ‘SAX‘:

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        'Use SAX
        ListBox1().Items.Clear()

        Dim xrReader As XmlReader = XmlReader.Create("BluRays.xml")

        Do While xrReader.Read()

            If xrReader.NodeType = XmlNodeType.Element AndAlso xrReader.Name = "Movie" Then

                ListBox1.Items.Add(xrReader.GetAttribute(0))

            End If

        Loop

    End Sub

All that happens here is we read the BluRays.xml file with the help of an XMLReader object, then I specified the key it should place into the ListBox. In this case it will scan through the file and show each movie’s name inside the ListBox. This is quite a small file, so it was read piece by piece instead of the whole file at the same time.

Reading XML Files with the Use of the DOM API

Add the following code behind the button labelled ‘DOM‘:

    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        'Use DOM

        ListBox1.Items.Clear()

        Dim xd As XmlDocument = New XmlDocument()

        xd.Load("BluRays.xml")

        Dim nlNodes As XmlNodeList = xd.GetElementsByTagName("Actor")

        For Each nNode As XmlNode In nlNodes

            ListBox1.Items.Add(nNode.InnerText)

        Next nNode

    End Sub

Here, with the aid of the XmlDocument object, you read the entire file and provide the Actors for each associated movie. The difference between SAX and DOM should be quite obvious now. With SAX I used the XmlReader object, which reads the file little by little, whereas with DOM I read the entire document into memory first, before providing the desired information.

Now the fun part starts!

Using XPath

XPath is best used with files that are online. Add the following code to the button labelled ‘XPath‘:

    Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click

        'Use Xpath
        Dim xpdDoc As XPathDocument
        Dim nsManager As XmlNamespaceManager
        Dim xpNav As XPathNavigator
        Dim xpIterator As XPathNodeIterator
        Dim xpNode As XPathNavigator

        ' Create a new XmlDocument
        xpdDoc = New XPathDocument("http://xml.movies.ncc-cla.com/ns/rss/1.0/BluRays.xml")

        ' Create navigator
        xpNav = xpdDoc.CreateNavigator()

        ' Set up namespace manager for XPath
        nsManager = New XmlNamespaceManager(xpNav.NameTable)
        nsManager.AddNamespace("movies", "http://xml.movies.ncc-cla.com/ns/rss/1.0")

        ' Get forecast with XPath
        xpIterator = xpNav.Select("/rss/channel/item/movies:movies", nsManager)

        While xpIterator.MoveNext()
            xpNode = xpIterator.Current

            Console.WriteLine("{0}: {1}, {2}", _
            xpNode.GetAttribute("Title", nsManager.DefaultNamespace), _
            xpNode.GetAttribute("Actor", nsManager.DefaultNamespace), _
            xpNode.GetAttribute("Type", nsManager.DefaultNamespace))

        End While

    End Sub

Now, here I created a new XML Document object, and set it to URL path. Once I have that, I created a Navigator object to help locating the specific elements I need to find in my XML document. With the Iterator object I literally iterate through each XML attribute in the file and display them. Be advised though that this is just an example so proper implementation may differ than what I have shown you here.

Conclusion

I hope you have enjoyed today’s article. I’ll see you around!

Hannes DuPreez
Ockert J. du Preez is a passionate coder and always willing to learn. He has written hundreds of developer articles over the years detailing his programming quests and adventures. He has written the following books: Visual Studio 2019 In-Depth (BpB Publications) JavaScript for Gurus (BpB Publications) He was the Technical Editor for Professional C++, 5th Edition (Wiley) He was a Microsoft Most Valuable Professional for .NET (2008–2017).

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