VB.Net - XML Processing

The Extensible Markup Language (XML) is a markup language much like HTML or SGML. This is recommended by the World Wide Web Consortium and available as an open standard.

The System.Xml namespace in the .Net Framework contains classes for processing XML documents. Following are some of the commonly used classes in the System.Xml namespace.

XML Parser APIs

The two most basic and broadly used APIs to XML data are the SAX and DOM interfaces.

• Simple API for XML (SAX) − Here, you register callbacks for events of interest and then let the parser proceed through the document. This is useful when your documents are large or you have memory limitations, it parses the file as it reads it from disk, and the entire file is never stored in memory.

• Document Object Model (DOM) API − This is World Wide Web Consortium recommendation wherein the entire file is read into memory and stored in a hierarchical (tree-based) form to represent all the features of an XML document.

SAX obviously can't process information as fast as DOM can when working with large files. On the other hand, using DOM exclusively can really kill your resources, especially if used on a lot of small files.

SAX is read-only, while DOM allows changes to the XML file. Since these two different APIs literally complement each other there is no reason why you can't use them both for large projects.

For all our XML code examples, let's use a simple XML file movies.xml as an input −

<?xml version = "1.0"?>

<collection shelf = "New Arrivals">
   <movie title = "Enemy Behind">
      <type>War, Thriller</type>
      <format>DVD</format>
      <year>2003</year>
      <rating>PG</rating>
      <stars>10</stars>
      <description>Talk about a US-Japan war</description>
   </movie>

   <movie title = "Transformers">
      <type>Anime, Science Fiction</type>
      <format>DVD</format>
      <year>1989</year>
      <rating>R</rating>
      <stars>8</stars>
      <description>A schientific fiction</description>
   </movie>

   <movie title = "Trigun">
      <type>Anime, Action</type>
      <format>DVD</format>
      <episodes>4</episodes>
      <rating>PG</rating>
      <stars>10</stars>
      <description>Vash the Stampede!</description>
   </movie>

   <movie title = "Ishtar">
      <type>Comedy</type>
      <format>VHS</format>
      <rating>PG</rating>
      <stars>2</stars>
      <description>Viewable boredom</description>
   </movie>
</collection>

Parsing XML with SAX API

In SAX model, you use the XmlReader and XmlWriter classes to work with the XML data.

The XmlReader class is used to read XML data in a fast, forward-only and non-cached manner. It reads an XML document or a stream.

Example 1

This example demonstrates reading XML data from the file movies.xml.

Take the following steps −

• Add the movies.xml file in the bin\Debug folder of your application.

• Import the System.Xml namespace in Form1.vb file.

• Add a label in the form and change its text to 'Movies Galore'.

• Add three list boxes and three buttons to show the title, type and description of a movie from the xml file.

• Add the following code using the code editor window.

Imports System.Xml
Public Class Form1
   Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
      ' Set the caption bar text of the form.   
      Me.Text = "howcodex.com"
   End Sub
   
   Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
      ListBox1().Items.Clear()
      Dim xr As XmlReader = XmlReader.Create("movies.xml")
      Do While xr.Read()
         If xr.NodeType = XmlNodeType.Element AndAlso xr.Name = "movie" Then
            ListBox1.Items.Add(xr.GetAttribute(0))
         End If
      Loop
   End Sub
   
   Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
      ListBox2().Items.Clear()
      Dim xr As XmlReader = XmlReader.Create("movies.xml")
      Do While xr.Read()
         If xr.NodeType = XmlNodeType.Element AndAlso xr.Name = "type" Then
            ListBox2.Items.Add(xr.ReadElementString)
         Else
            xr.Read()
         End If
      Loop
   End Sub
   
   Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
      ListBox3().Items.Clear()
      Dim xr As XmlReader = XmlReader.Create("movies.xml")
      Do While xr.Read()
         If xr.NodeType = XmlNodeType.Element AndAlso xr.Name = "description" Then
            ListBox3.Items.Add(xr.ReadElementString)
         Else
            xr.Read()
         End If
      Loop
   End Sub
End Class

Execute and run the above code using Start button available at the Microsoft Visual Studio tool bar. Clicking on the buttons would display, title, type and description of the movies from the file.

The XmlWriter class is used to write XML data into a stream, a file or a TextWriter object. It also works in a forward-only, non-cached manner.

Example 2

Let us create an XML file by adding some data at runtime. Take the following steps −

• Add a WebBrowser control and a button control in the form.

• Change the Text property of the button to Show Authors File.

• Add the following code in the code editor.

Imports System.Xml
Public Class Form1
   Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
      ' Set the caption bar text of the form.   
      Me.Text = "howcodex.com"
   End Sub
   Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
      Dim xws As XmlWriterSettings = New XmlWriterSettings()
      xws.Indent = True
      xws.NewLineOnAttributes = True
      Dim xw As XmlWriter = XmlWriter.Create("authors.xml", xws)
      xw.WriteStartDocument()
      xw.WriteStartElement("Authors")
      xw.WriteStartElement("author")
      xw.WriteAttributeString("code", "1")
      xw.WriteElementString("fname", "Zara")
      xw.WriteElementString("lname", "Ali")
      xw.WriteEndElement()
      xw.WriteStartElement("author")
      xw.WriteAttributeString("code", "2")
      xw.WriteElementString("fname", "Priya")
      xw.WriteElementString("lname", "Sharma")
      xw.WriteEndElement()
      xw.WriteStartElement("author")
      xw.WriteAttributeString("code", "3")
      xw.WriteElementString("fname", "Anshuman")
      xw.WriteElementString("lname", "Mohan")
      xw.WriteEndElement()
      xw.WriteStartElement("author")
      xw.WriteAttributeString("code", "4")
      xw.WriteElementString("fname", "Bibhuti")
      xw.WriteElementString("lname", "Banerjee")
      xw.WriteEndElement()
      xw.WriteStartElement("author")
      xw.WriteAttributeString("code", "5")
      xw.WriteElementString("fname", "Riyan")
      xw.WriteElementString("lname", "Sengupta")
      xw.WriteEndElement()
      xw.WriteEndElement()
      xw.WriteEndDocument()
      xw.Flush()
      xw.Close()
      WebBrowser1.Url = New Uri(AppDomain.CurrentDomain.BaseDirectory + "authors.xml")
   End Sub
End Class

• Execute and run the above code using Start button available at the Microsoft Visual Studio tool bar. Clicking on the Show Author File would display the newly created authors.xml file on the web browser.

Parsing XML with DOM API

According to the Document Object Model (DOM), an XML document consists of nodes and attributes of the nodes. The XmlDocument class is used to implement the XML DOM parser of the .Net Framework. It also allows you to modify an existing XML document by inserting, deleting or updating data in the document.

Following are some of the commonly used methods of the XmlDocument class −

Example 3

In this example, let us insert some new nodes in the xml document authors.xml and then show all the authors' first names in a list box.

Take the following steps −

• Add the authors.xml file in the bin/Debug folder of your application( it should be there if you have tried the last example)

• Import the System.Xml namespace

• Add a list box and a button control in the form and set the text property of the button control to Show Authors.

• Add the following code using the code editor.

Imports System.Xml
Public Class Form1
   Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
      ' Set the caption bar text of the form.   
      Me.Text = "howcodex.com"
   End Sub
   
   Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
      ListBox1.Items.Clear()
      
      Dim xd As XmlDocument = New XmlDocument()
      xd.Load("authors.xml")
      
      Dim newAuthor As XmlElement = xd.CreateElement("author")
      newAuthor.SetAttribute("code", "6")
      
      Dim fn As XmlElement = xd.CreateElement("fname")
      fn.InnerText = "Bikram"
      newAuthor.AppendChild(fn)
      
      Dim ln As XmlElement = xd.CreateElement("lname")
      ln.InnerText = "Seth"
      newAuthor.AppendChild(ln)
      xd.DocumentElement.AppendChild(newAuthor)
      
      Dim tr As XmlTextWriter = New XmlTextWriter("movies.xml", Nothing)
      tr.Formatting = Formatting.Indented
      xd.WriteContentTo(tr)
      tr.Close()
      
      Dim nl As XmlNodeList = xd.GetElementsByTagName("fname")
      
      For Each node As XmlNode In nl
         ListBox1.Items.Add(node.InnerText)
      Next node
   End Sub
End Class

• Execute and run the above code using Start button available at the Microsoft Visual Studio tool bar. Clicking on the Show Author button would display the first names of all the authors including the one we have added at runtime.