XML namespace prefixes in MSXML

If you're working with XSL or a similar technology that makes use of XML namespace prefixes using the Microsoft XML DOM you'll likely run into problems if you try to do anything more than just load in a file.

Adding elements

The W3 DOM specification includes a createElementNS method for creating an element scoped within a namespace however MSXML doesn't. You can create an element with a prefix using createElement but this doesn't correctly register the namespace of the node and you'll get a schema error something like:

msxml3.dll: Keyword xsl:stylesheet may not contain xsl:include.

In order to create an element and register it correctly you have to use createNode instead which takes node type (1 for an element), node name and namespace URI as arguments e.g.

Set ndIncl = xslDoc.createNode(1, "xsl:include",
"http://www.w3.org/1999/XSL/Transform")

Using XPath

Similar to the createElement problem, even if you've only loaded an XSL document you won't be able to use XPath to query it because oddly the namespaces aren't automatically registered with XPath e.g.

Set nlTemps = xslDoc.documentElement.selectNodes("/xsl:stylesheet/xsl:template")

yields the following error:

msxml3.dll: Reference to undeclared namespace prefix: 'xsl'.

To get this to play ball you have to set the "SelectionNamespaces" second-level property which takes a space delimited list of namespace definitions using a setProperty call of the form:

xslDoc.setProperty "SelectionNamespaces",
"xmlns:xsl='http://www.w3.org/1999/XSL/Transform'"

Links

• createElementNS @ W3C
• createNode @ MSDN
• SelectionNamespaces propterty @ MSDN

Working with XML - Part 3 - Formatting XML using XSL

• Working with XML - Part 1 - Using the DOM
• Working with XML - Part 2 - Using XPath to query XML
• Working with XML - Part 3 - Formatting XML using XSL

XSL is an language for transforming XML into different formats, written in XML using specific elements and attributes. To achieve the transform you simply load you XML and XSL into DOM objects then call transformNode on the XML document passing it your XSL document and it will return the transformed output.

XSL is effectively a procedural programming language. It has conditional and looping logic structures as well as variables and callable "functions". I'd recommend reading XSL @ W3SChools as this explains the basics well.

Recap

In parts one and two we looked at loading a sample bit of XML and how to select certain nodes from it using XPath.

<?xml version="1.0" ?>
<library>

  <authors>
     <author id="12345">
        <name>Charles Dickens</name>
     </author>
     <author id="23456">  
        <name>Rudyard Kipling</name>

     </author>        
  </authors>
  <books>
     <book>
        <title>Great Expectations</title>
        <author>12345</author>

     </book>
     <book>
        <title>The Jungle Book</title>
        <author>23456</author>
     </book>

  </books>
</library>

Using XSL we can convert this XML document into, for example, HTML to send to a web browser. We could also transform it into SQL INSERT statements for adding the data to a database.

HTML Output - Listing Data

Here's an example of how you could output the records from the sample XML:

<?xml version="1.0" ?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

<!-- more stuff to go here -->

</xsl:stylesheet>

We start with the stylesheet element which specifies the XML namespace "xsl" which is used as the prefix for all the special XSL elements.

<xsl:template match="/">
   <html>
      <head>
         <title>Library</title>
      </head>
      <body>
         <h1>Library</h1>
         <xsl:apply-templates select="authors/author" />
      </body>
   </html>
</xsl:template>

Next we have a template element with a match attribute equal to "/". This template matches the root of the XML document so the transform starts here outputting the contents of the element. The apply-templates element then tells the transform to apply the appropriate templates to the nodes returned by the value of the select attribute which you'll notice is XPath.

<xsl:template match="author">
   <h2><xsl:value-of select="name" /> (<xsl:value-of select="@id" />)</h2>
   <p>Books by this author:</p>
   <table>
      <tr>
        <th>Title</th>
      </tr>
      <xsl:apply-templates select="/library/books/book[author = current()/@id]" />
   </table>
</xsl:template>

This template will match the author nodes selected so output will pass here at this point - similar to a function call in a normal programming language. Here value-of elements output the value of the nodes identified in their select attributes, XPath again.

The next apply-templates element selects all the book elements with an author child element whose value is equal to the current author element's id attribute. It uses the current() XSL function to get at the element being tranformed by the template.

<xsl:template match="book">
   <tr>
      <td><xsl:value-of select="title" /></td>
   </tr>
</xsl:template>

Finally the titles of the books are writen out on table rows so what you end up with after calling transformNode is this:

<html>
  <head>
    <title>Library</title>
  </head>
  <body>
    <h1>Library</h2>
    <h2>Charles Dickens (12345)</h2>
    <p>Books by this author:</p>
    <table>
      <tr>
        <th>Title</th>
      </tr>
      <tr>
        <td>Great Expectations</td>
      </tr>
    </table>
    <h2>Rudyard Kipling (23456)</h2>
    <p>Books by this author:</p>
    <table>
      <tr>
        <th>Title</th>
      </tr>
      <tr>
        <td>The Jungle Book</td>
      </tr>
    </table>
  </body>
</html>

Using the output element

XHTML compliant output

XSL has an HTML output mode that you can specify by adding this to the top of your stylesheet, before any template elements:

<xsl:output mode="html" />

However the Microsoft.XMLDOM will mess around with your tags if you use this so if you want your output to be XHTML compliant you need to use the XML output mode instead thusly:

<xsl:output method="xml" omit-xml-declaration="yes" />

Outputting a DOCTYPE

If you want to output a DOCTYPE (which you need to get IE to obey the CSS box model properly) you add a few attributes to your output element:

<xsl:output 
   method="xml" 
   omit-xml-declaration="yes"
   doctype-public="-//W3C//DTD XHTML 1.0 Strict//EN"
   doctype-system="http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
/>

which will produce:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

For more info on the output element visit output @ W3Schools.

Useful XSL snipets

Alternating row classes on tables

<xsl:template match="book">
   <tr>
      <xsl:attribute name="class">
         <xsl:choose>
            <xsl:when test="(position() mod 2) = 0">even</xsl:when>
            <xsl:otherwise>odd</xsl:otherwise>
         </xsl:choose>
      </xsl:attribute>
      <td><xsl:value-of select="title" /></td>
   </tr>
</xsl:template>

Template applicability

You can make nodes of the same type product different output by changing the match attribute of your template:

<xsl:template match="book[author = 12345]">
   <tr>
      <td class="highlight"><xsl:value-of select="title" /></td>
   </tr>
</xsl:template>

<xsl:template match="book">
   <tr>
      <td><xsl:value-of select="title" /></td>
   </tr>
</xsl:template>

Here we're applying a highlight class to book rows by author 12345.

Links

• XSL @ W3C
• XSL @ W3Schools
• Zvon XSL Tutorial

Working with XML - Part 2 - Using XPath to query XML

• Working with XML - Part 1 - Using the DOM
• Working with XML - Part 2 - Using XPath to query XML
• Working with XML - Part 3 - Formatting XML using XSL

XPath is a simple language for querying XML documents in order to retrieve nodes matching particular criteria. There are some good references and tutorials out there to help you get to grips with the basics; i'd recommend reading XPath @ W3Schools for starters and then running through the Zvon XPath Tutorial before reading on.

XSL, which the next part of this post is about, makes much use of XPath so you need to get up to speed with it before you venture into XSL.

Recap

In part one we loaded the following XML in to the DOM and performed various operations with DOM properties and methods. To use XPath there are only two methods selectNodes which returns a node list and selectSingleNode which returns one node.

<?xml version="1.0" ?>
<library>
  <authors>
     <author id="12345">
        <name>Charles Dickens</name>
     </author>
     <author id="23456">  
        <name>Rudyard Kipling</name>
     </author>        
  </authors>
  <books>
     <book>
        <title>Great Expectations</title>
        <author>12345</author>
     </book>
     <book>
        <title>The Jungle Book</title>
        <author>23456</author>
     </book>
  </books>
</library>

XPath allows you to do some quite complex data selection and analysis using a mixture of path syntax, axes, predicates and functions. Having said that i had quite a hard time finding decent examples of doing some quite simple stuff.

Important - Setting the SelectionLanguage property

If you're using the Microsoft XMLDOM COM component you need to set the SelectionLanguage property of the DOM document to "XPath" otherwise you'll get some very odd results - you do this as follows:

xmlDoc.setProperty "SelectionLanguage", "XPath"

Example 1 - Selecting nodes and checking return value

'a single node
strXPath = "/library/authors"
Set ndAuthors = xmlDoc.documentElement.selectSingleNode(strXPath)

'This test checks whether authors was found or not...
If ndAuthors Is Nothing Then
  'error not found!
Else
  'do something with authors
End If

'multiple nodes
strXPath = "/library/authors/author"
Set nlAuthors = xmlDoc.documentElement.selectNodes(strXPath)

'This test checks whether author nodes were found or not...
If nlAuthors.Length = 0 Then
  'error not found!
Else
  For Each ndAuthor In nlAuthors
     'do something with nodes
  Next
End If

If you ran through the Zvon XPath tutorial earlier you should now be able to do some basic selecting of nodes using the two methods i've just shown you.

In the next few examples i'm going to run through some of the things you'll probably want to do but tutorials like the Zvon one don't cover.

Example 2 - Predicates and Axes

Selecting the author element with id "12345"...

strXPath = "/library/authors/author[@id='12345']"
Set ndNode = xmlDoc.documentElement.selectSingleNode(strXPath)

Only selecting the author's name...

strXPath = "/library/authors/author[@id='12345']/name"
Set ndNode = xmlDoc.documentElement.selectSingleNode(strXPath)

Titles of books written by that author...

strXPath = "/library/books/book[author='12345']/title"
Set nlNodes = xmlDoc.documentElement.selectNodes(strXPath)

Example 3 - Functions

Simple counting of nodes...

strXPath = "count(/library/authors/author)"
Set ndCount = xmlDoc.documentElement.selectSingleNode(strXPath)

Combining count with the ancestor axis allows you to select nodes of a particular depth...

strXPath = "//*[count(ancestor::*) > 2]"
Set nlDeepNodes = xmlDoc.documentElement.selectNodes(strXPath)

Books with "The" in their title...

strXPath = "/library/books/book[contains(title,'The')]"
Set nlNodes = xmlDoc.documentElement.selectNodes(strXPath)

Example 4 - Common tasks

Remove nodes that match certain criteria...

strXPath = "/library/books/book[contains(title,'The')]"
Set nlNodes = xmlDoc.documentElement.selectNodes(strXPath)
For Each ndNode In nlNodes
   ndNode.parentNode.removeChild ndNode
Next

Links

• XPath @ W3Schools
• Zvon XPath Tutorial
• XPath @ W3C