XML Reference Guide

Mar 14, 2003

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With XML so fundamental in Microsoft's .NET framework, it should come as no surprise that the ability to manipulate an XML document is built right into the system. In fact, although DOM Level 2.0 doesn't include a standard way to create or save a document, .NET makes it easy. In this section, we'll get a feel for how these manipulations work by using Visual Basic .NET to load a simple document, make some changes to it, and then save it back out to a file. (We'll be running directly from the command line, so if you're not familiar with GUI programming, don't worry.)

Consider, for example, the following sample file, candy.xml:

<?xml version="1.0"?>
<candy>
  <product>Mints</product>
  <product>Chocolate</product>
  <product>Circus Peanuts</product>
</candy>

Using .NET, you can create a Document out of it by either feeding the XML directly to the parser, as in:

Option Strict Off

Imports System
Imports System.IO
Imports System.Xml

public class ShowFile

  public shared sub Main()

     Dim doc as XmlDocument = new XmlDocument()
     doc.LoadXml("<candy>  <product>Mints</product>" _
           & "<product>Chocolate</product>" _
           & "<product>Circus Peanuts</product></candy>")
     
  end sub
end class

or reading a file from a URL, as in:

...
  public shared sub Main()

     Dim doc as XmlDocument = new XmlDocument()
     doc.Load("http://www.example.com/candy.xml")
     
  end sub
...

Once you create the Document (or in this case, the XmlDocument), you can manipulate it using the .NET binding of the DOM interface, as in:

Option Strict Off

Imports System
Imports System.IO
Imports System.Xml

public class ShowFile

  public shared sub Main()

     Dim doc as XmlDocument = new XmlDocument()
     doc.Load("http://www.example.com/candy.xml")
     
     dim root as XmlElement = doc.DocumentElement
     Console.WriteLine("The root element is " + root.Name + ".")

     dim children as XmlNodeList = root.ChildNodes
     Console.Write("There are ")
     Console.Write(children.Count)
     Console.WriteLine (" child elements.")
     Console.WriteLine("They are: ")

     if root.HasChildNodes then

         for each thisChild as XmlNode in root.ChildNodes

             if thisChild.NodeType = XmlNodeType.Element then
                  Dim theText as XmlNode
                  theText = thisChild.FirstChild
                  Console.WriteLine(thisChild.Name + ": " + theText.Value)
             end if

        next

     end if

  end sub
end class

First we're getting a reference to the root of the actual Document by asking for the DocumentElement. (Note that in Java, we would use getDocumentElement() rather than referencing the member directly.) We can then access the Name property of that element.

Next, we get an XmlNodeList of all of the child nodes of the root element. That list has a Count property we can use to determine how many children their are. You could, for example, then use this number to iterate through each node directly, but instead let's look at doing it implicitly by calling the objects themselves.

If the root node has children, we can iterate through them by setting thisChild to point to each one in turn. If a node is an element, we can assume (in this case) that it has a text child, which we can then access using the FirstChild member. From there, we can simply print out the values.

We can also use the DOM interface to add information to the Document, as in:

...
                  Console.WriteLine(thisChild.Name + ": " + theText.Value)
             end if

        next

     end if
 
     Dim products as XmlNodeList = root.GetElementsByTagName("product")
     Dim productNode as XmlNode

     Dim productNum as integer
     for productNum = 0 to (products.Count - 1)

        productNode = products.item(productNum)
        Dim attr as XmlAttribute = doc.CreateAttribute("productNumber")
        attr.Value = "Product "&productNum

        Dim productElement as XmlElement = productNode
        productElement.setAttributeNode(attr)

        Dim productName as String = productElement.FirstChild.Value
        productElement.FirstChild.Value = productName.toUpper

        Dim updateElement as XmlElement = doc.CreateElement("updated")
        Dim rightNow as DateTime
        
        Dim updateText as XmlNode = doc.CreateTextNode(rightNow.Now)

        updateElement.AppendChild(updateText)
        productElement.AppendChild(updateElement)

        Console.WriteLine(productNum)
     next

  end sub
end class

What we want to do is add an attribute and an element to each product element, so first we can get all of the product elements using GetElementsByTagName(). Next, we can loop through each of them using the Count value, as I mentioned earlier.

For each node, we can create an attribute (using CreateAttribute()) and then assign it a value. From there, you can cast the Node to an Element and assign the Attribute to it. You can also assign the value directly, using:

productElement.SetAttribute("productNumber", "", "Product "&productNum)

Next, we're altering the product name directly by assigning the Text node child of the element a new value.

Finally, we'll make a new Element by asking the Document object to create the it -- that's it's job, according to the DOM specification. We can then create a Text node and append it to the new element, and then append the new element to the product.

Because persisting, or saving, an XML document wasn't part of the DOM specification, every implementation handles it differently, if it handles it at all. For example, in most Java implementations, the easiest way to save an XML Document to a file is to perform an XSL identity tranformation. Fortunately, .NET doesn't make you go through such convolutions, enabling you to simple save() the Document:

...
        Console.WriteLine(productNum)
     next

     doc.save("file:///c:|/output/candyout.xml")
     Console.WriteLine("Done.")

  end sub
end class

In this case, the end result is a file that shows the appropriate changes, as in:

<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>

Note that I've added some text nodes to make the changes easier to see, but the data is the same.

Note that althought I've done these examples in Visual Basic .NET, the core implementation is the same for C# and C++, though of course the syntax is different.

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