Extend your XSLT with custom functions

Sometimes the built-in functions aren't enough. Fortunately the XSLT-parser allows us to extend it with our own C# or Javascript code.
The following is an entire xslt-file, which is split up in parts to explain what is going on. 

Here we're telling the parser to create a new xml namespace for our functions.

<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet
version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxml="urn:schemas-microsoft-com:xslt"
xmlns:umbraco.library="urn:umbraco.library"
xmlns:myfunctionlib="urn:myfunctionlib"
exclude-result-prefixes="msxml umbraco.library myfunctionlib">
  <xsl:output method="xml" omit-xml-declaration="yes"/>
   <xsl:param name="currentPage"/>

Then it's time to tell the XSLT-parser to tie the script-block to our custom xml namespace (myfunctionlib). Furthermore we're using the msxml:assembly-statement to load and prepare the System.Web namespace for use. The msxml:using statement makes the System.Web classes and methods available:

<msxml:script implements-prefix="myfunctionlib" language="C#">
<msxml:assembly name="System.Web"/>
<msxml:using namespace="System.Web"/>
<![CDATA[
public string convertText(string text)
{
    string decodedStr = HttpUtility.HtmlDecode(text);
    return decodedStr;
}
]]>
</msxml:script>

The C# codeblock itself is surrounded by a <![CDATA-statement.. This is not a requirement, but generally a good idea, because it prevents the XSLT-parser from thinking it is dealing with XSLT-markup itself

  <xsl:template match="/">
    <xsl:value-of select="umbraco.library:ChangeContentType('text/xml')"/>
    <xsl:text disable-output-escaping="yes">&lt;?xml version="1.0" encoding="UTF-8"?&gt;</xsl:text>
      <Root>
          <xsl:apply-templates select="$currentPage/descendant::node/node [string(data [@alias='umbracoNaviHide']) != '1']">
        </xsl:apply-templates>
      </Root>
  </xsl:template>
  <xsl:template match="node">
      <Story>
          <xsl:value-of select="myfunctionlib:convertText(string(./data [@alias='bodyText']))" disable-output-escaping="yes"/>
      </Story>
  </xsl:template>
</xsl:stylesheet>

Finally we're ready to use our custom function (highlighted with bold)