XML Reference Guide

Mar 14, 2003

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In the world of XML, it's natural to think of XML in terms of two contexts: DOM and SAX. DOM is flexible, in that it provides the ability to navigate around the document tree and make changes, but SAX is fast, in that it doesn't load the entire document into memory, looking instead at just a single node at a time in a forward-only, read-only manner.

But .NET doesn't readily support SAX. Why? Because Microsoft found a way to provide much of the same benefit without the complex programming that SAX requires. The XmlReader object is like a cross between a DOM document and a read-only, forward-only ADO cursor. It allows you to easily access the properties of the current node (including some measure of positioning within the document), but also provides the speed and power of a lightweight object. In fact, unless you're going to make changes to the document tree itself, you'll probably find XmlReader much easier and more convenient to use than the XmlDocument objects.

In DOM and .NET, we looked at what it takes to navigate a document, so let's look at what it takes using an XmlReader implementation. Let's say that we wanted to look at a tidied up version of the candy output:

<candy>
  <product productNumber="Product 0">
        MINTS
        <updated>1/11/2004 9:37:35 PM</updated>
  </product>
  <product productNumber="Product 1">
        CHOCOLATE
        <updated>1/11/2004 9:37:35 PM</updated>
  </product>
  <product productNumber="Product 2">
        CIRCUS PEANUTS
        <updated>1/11/2004 9:37:35 PM</updated>
  </product>
</candy>

We'd go about it by creating an XmlReader object and looping through each node:

Imports System
Imports System.IO
Imports System.Xml

public class ReaderSample

  public shared sub Main()

    dim reader as XmlTextReader = new XmlTextReader("candyout.xml")

    while reader.Read()
       Indent(reader.Depth)

       Console.WriteLine("Name: [{0}] Type: [{1}] Value: [{2}] ", _
                 reader.Name, reader.NodeType, reader.Value)

    end while

  end sub

  public shared sub Indent(depth as Integer)
    dim i as Integer
    for i = 0 to depth
      Console.Write("  ")
    next
  end sub

end class

First we're creating the XmlReader object. Note that XmlReader is an abstract class; you must use one of the three overriding classes, XmlTextReader (for serialized text, such as a file), XmlNodeReader (for documents that have already been parsed into an XmlNode object), and XmlValidatingReader, for enforcing DTDs and XML Schemas.

When using an XmlReader, you look at one node at a time, using the Read() method. Each time you call the Read() method, it advances to the next node. From there, you can access information about the node through the members of the class, such as Value and Name. Note that at any given time, the reader object represents the current node rather than the overall document.

When the application gets to the end of the document, Read() returns false and it exits the loop.

The output at this stage is simple:

  Name: [candy] Type: [Element] Value: []
    Name: [] Type: [Whitespace] Value: [
  ]
    Name: [product] Type: [Element] Value: []
      Name: [] Type: [Text] Value: [
        MINTS
        ]
      Name: [updated] Type: [Element] Value: []
        Name: [] Type: [Text] Value: [1/11/2004 9:37:35 PM]
      Name: [updated] Type: [EndElement] Value: []
      Name: [] Type: [Whitespace] Value: [
  ]
    Name: [product] Type: [EndElement] Value: []
    Name: [] Type: [Whitespace] Value: [
  ]
    Name: [product] Type: [Element] Value: []
      Name: [] Type: [Text] Value: [
        CHOCOLATE
        ]
      Name: [updated] Type: [Element] Value: []
        Name: [] Type: [Text] Value: [1/11/2004 9:37:35 PM]
      Name: [updated] Type: [EndElement] Value: []
      Name: [] Type: [Whitespace] Value: [
  ]
    Name: [product] Type: [EndElement] Value: []
    Name: [] Type: [Whitespace] Value: [
  ]
    Name: [product] Type: [Element] Value: []
      Name: [] Type: [Text] Value: [
        CIRCUS PEANUTS
        ]
      Name: [updated] Type: [Element] Value: []
        Name: [] Type: [Text] Value: [1/11/2004 9:37:35 PM]
      Name: [updated] Type: [EndElement] Value: []
      Name: [] Type: [Whitespace] Value: [
  ]
    Name: [product] Type: [EndElement] Value: []
    Name: [] Type: [Whitespace] Value: [
]
  Name: [candy] Type: [EndElement] Value: []

We can clean it up slightly by telling the application to ignore whitespace:

...
    dim reader as XmlTextReader = new XmlTextReader("candyout.xml")

    reader.WhitespaceHandling = WhitespaceHandling.None

    while reader.Read()
...

Now let's look at the results:

  Name: [candy] Type: [Element] Value: []
    Name: [product] Type: [Element] Value: []
      Name: [] Type: [Text] Value: [
        MINTS
        ]
      Name: [updated] Type: [Element] Value: []
        Name: [] Type: [Text] Value: [1/11/2004 9:37:35 PM]
      Name: [updated] Type: [EndElement] Value: []
    Name: [product] Type: [EndElement] Value: []
    Name: [product] Type: [Element] Value: []
      Name: [] Type: [Text] Value: [
        CHOCOLATE
        ]
      Name: [updated] Type: [Element] Value: []
        Name: [] Type: [Text] Value: [1/11/2004 9:37:35 PM]
      Name: [updated] Type: [EndElement] Value: []
    Name: [product] Type: [EndElement] Value: []
    Name: [product] Type: [Element] Value: []
      Name: [] Type: [Text] Value: [
        CIRCUS PEANUTS
        ]
      Name: [updated] Type: [Element] Value: []
        Name: [] Type: [Text] Value: [1/11/2004 9:37:35 PM]
      Name: [updated] Type: [EndElement] Value: []
    Name: [product] Type: [EndElement] Value: []
  Name: [candy] Type: [EndElement] Value: []

Notice that the structure is much like that of the XML document itself, with both the start and the end of each element noted, and the "depth" easily accessible. Both of these features can be achieved with SAX -- but not easily.

We can also look at the attributes on each element:

...
      while reader.Read()
      Indent(reader.Depth)

      Console.WriteLine("Name: [{0}] Type: [{1}] Value: [{2}] ", _
                 reader.Name, reader.NodeType, reader.Value)

      if reader.HasAttributes then
        Indent(reader.Depth)
        Indent(reader.Depth)
        Console.Write("Attributes: ")
        Dim i As Integer
        For i = 0 To reader.AttributeCount - 1
            reader.MoveToAttribute(i)
            Console.Write(" {0}=[{1}]", reader.Name, reader.Value)
        Next i
        reader.MoveToElement()
        Console.WriteLine("")
      end if

    end while

  end sub
...

If attributes exist on the current node, we can output their value by accessing them directly by index. (You can also pair the MoveToFirstAttribute() and MoveToNextAttribute() methods.) When you move to an attribute, that attribute becomes the current node, so members such as Name and Value refer to that attribute. When you're done with those attributes, you can use the MoveToElement() method to return to the element that contains the attributes.

The XmlReader classes have a variety of useful methods we haven't even touched on, but this should be enough to get you started. If you're only reading XML data, chances are an XmlReader is the way to go.

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