VB.Net - XML processing

Extensable tag language (XML) is a tag language that is very similar to HTML or SGML. This is recommended by the World Wide Web Alliance and can be used as an open standard.

. T he System.Xml namespace in the Net framework contains classes for working with XML documents. Here are some of the classes commonly used in the System.Xml namespace.

The API of the XML parser

The two most basic and widely used APIs for XML data are the SAX and DOM interfaces.

XML's Simple API (SAX): Here you register a callback for the event of interest and let the parser continue working on the document. This is useful when the document is large or memory-limited, and it resolves files when they are read from disk, and the entire file is never stored in memory.

Document Object Model (DOM) API: This is recommended by the World Wide Web Federation, where the entire file is read into memory and stored in a hierarchical (tree-based) form to represent all the characteristics of an XML document.

SAX obviously can't process information as quickly as the DOM when using large files. On the other hand, using DOM can really kill your resources, especially if used for many small files.

SAX is read-only, and the DOM allows changes to XML files. Since these two different APIs are literally complementary, there's no reason why you can't use them for two big projects.

For all the XML code examples, let's use a simple XML file .xml as 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 uses the SAX API

In the SAX model, XmlReader and XmlWriter classes are used to process XML data.

The XmlReader class is used to read XML data quickly, forward and non-cached only. It reads XML documents or streams.

Example 1

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

Follow these steps:

• Add the .xml files to the application's bin sdbug folder.

• Import system.vb.Xml namespaces in the Form1 file.
  

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

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

• Use the code editor window to add the following code.

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 = "tutorialspoint.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

Use the Start button on the Microsoft Visual Studio toolbar to execute and run the code above. C licking the button displays the title, type, and description of the movie in the file.

The XmlWriter class is used to write XML data to streams, files, or TextWriter objects. I t also works in a forward-only, non-cached manner.

Example 2

Let's create an XML file by adding some data at runtime. F ollow these steps:

• Add WebBrowser controls and button controls to the form.

• Change the Text property of the button to show the author file.

• Add the following code to 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 = "tutorialspoint.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

Use the Start button on the Microsoft Visual Studio toolbar to execute and run the code above. C licking show author files will display the newly created autos file on .xml browser.

Use the DOM API to resolve XML

Depending on the document object model (DOM), the XML document consists of nodes and node properties. T he XmlDocument class is the XML DOM parser used to implement the .Net framework. It also allows you to modify existing XML documents by inserting, deleting, or updating data in the document.

Here are some common methods for the XmlDocument class:

Example 3

In this example, let's insert some new nodes .xml the xml document auss, and then display the names of all the authors in the list box.

Follow these steps:

• Add the .xml file to the application's bin/Debug folder (where you should be if you've tried the last example)

• Import the System.Xml namespace

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

• Use the code editor to add the following code.

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 = "tutorialspoint.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

Use the Start button on the Microsoft Visual Studio toolbar to execute and run the code above. C licking the Show Authors button will display the names of all authors, including those we added at runtime.