The System.Xml Namespace

You'll use the System.Xml namespace for processing XML data. This namespace supports an ever-expanding host of XML standards, such as the following:

• XML 1.0

• Namespaces

• Schemas

• XSL/T

• SOAP 1.1

The System.Xml namespace contains classes that represent the various XML elements, similar to the objects provided by the MSXML library for Visual Basic 6.0 developers. For example, you'll find an XmlDocument class, an XmlEntity class, and an XmlNode class. You can also use XmlValidatingReader in order to read XML and validate it against a Document Type Definition (DTD), XML Data Reduced (XDR) schema, or an XML Schema Definition (XSD).

The System.Xml namespace includes reader and writer classes that provide fast, forward-only reading and writing of XML streams. Back in the System.Data discussion, you learned about the DataReader class, which is used for fast forward-only data access. XmlTextReader provides the same basic functionality against an XML stream.

When you use a reader object to read XML, you can use properties of the reader to determine each node type and act accordingly. A writer class has methods such as WriteCData, WriteDocType, and WriteNode in order to create an XML document, element by element.

A System.Xml Example

The example in Listing 4.5 combines several of the namespaces covered in this chapter. First, it uses System.Data classes to read data from SQL Server. Next, it uses System.IO classes to output that data into an XML file. Finally, it reopens the original XML file and uses System.Xml to read the contents of the file and copy the data into a text box. Here's the code:

Listing 4.5. Use an XmlTextReader Class to Parse an XML Document

Imports System.IO
Imports System.XML
Imports System.Text
Imports System.Data.OleDb

Private Sub XMLWrite()
  Dim strConn As String
  Dim strSQL As String
  Dim strName As String
  Dim strXML As String
  Dim sw As StreamWriter
  Dim sr As XmlTextReader
  Dim sb As New StringBuilder(256)
  Dim da As OleDbDataAdapter
  Dim ds As DataSet

  strConn = "Provider=sqloledb;" & _
   "Data Source=(local);" & _
   "Initial Catalog=Northwind;" & _
   "User ID=sa"
  strSQL = "SELECT ProductID, ProductName FROM Products"

  da = New OleDbDataAdapter(strSQL, strConn)
  ds = New DataSet()
  da.Fill(ds)
  strName = "c:	est.xml"

  sw = File.CreateText(strName)
  sw.WriteLine(ds.GetXml)
  sw.Close()

  sr = New XmlTextReader(strName)
  sr.WhitespaceHandling = WhitespaceHandling.None

  ' Parse the file and display each of the nodes.
  While sr.Read()
    Select Case sr.NodeType
      Case XmlNodeType.Element
        sb.AppendFormat("<{0}>", sr.Name)
      Case XmlNodeType.Text
        sb.Append(sr.Value)
      Case XmlNodeType.XmlDeclaration
        sb.Append("<?xml version='1.0'?>")
      Case XmlNodeType.Document
      Case XmlNodeType.DocumentType
        sb.AppendFormat("<!DOCTYPE {0}[{1}]", _
         sr.Name, sr.Value)
      Case XmlNodeType.EndElement
        sb.AppendFormat("</{0} >", sr.Name)
    End Select
  End While

  txtXML.Text = sb.ToString()
End Sub