implementing evaluate(XPath) in msxml - xslt

Question

I'm writing an experiment where I take an XML file, that has XPaths embedded in it, and try to process it against another XML file with data in it, where the XPaths refer to elements within some predefined nodeset inside the data....basically binding a view to a list of data.
I've basically got it working, except how to evaluate the XPaths themselves, clearly I can do it in saxon with 3.0 (maybe I should try there first), but it would be initially convenient if this worked in msxml. I've read stuff about "extensions" and embedding javascript...but I can't really see how it would work (it didnt work for me).
any ideas?
(I could make the xslt create an xslt that creates the output, but this is a rough and ready proof of concept, and that might make my head hurt).
I can if necessary create an explicit example, but my actual scenario is quite convoluted.

Answer 1

Within .NET XPathNavigator has an Evaluate method: https://learn.microsoft.com/en-us/dotnet/api/system.xml.xpath.xpathnavigator.evaluate?view=netcore-3.1. You can expose it from an extension object to XslCompiledTransform.
The Mvp.Xml library does this to expose a dyn2:evaluate extension function in the namespace xmlns:dyn2="http://gotdotnet.com/exslt/dynamic" to its wrapper MvpXslTransform class around XslCompiledTransform.
The library is available on NuGet in a .NET standard 2.0 compatible (i.e. both .NET framework and .NET Core compatible) package: https://www.nuget.org/packages/Mvp.Xml.NetStandard.
Simplest code would be (using Mvp.Xml.Common.Xsl;):
var processor = new MvpXslTransform();
processor.SupportedFunctions = Mvp.Xml.Exslt.ExsltFunctionNamespace.GdnDynamic;
processor.Load("XSLTFile1.xslt");
processor.Transform(new XmlInput("XMLFile1.xml"), null, new XmlOutput(Console.Out));
where the stylesheet uses e.g. dyn2:evaluate(., path) with xmlns:dyn2="http://gotdotnet.com/exslt/dynamic" where the path child element of the context node contains an XPath expression.
The source code is at https://github.com/keimpema/Mvp.Xml.NetStandard/blob/master/library/Mvp.Xml/Exslt/GDNDynamic.cs.
In the .NET framework you can also embed .NET code directly in the XSLT and XslCompiledTransform will compile and run it with the proper XsltSettings (new XsltSettings() { EnableScript = true }):
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:msxsl="urn:schemas-microsoft-com:xslt"
xmlns:my-ext="http://example.com/my-ext"
exclude-result-prefixes="msxsl my-ext">
<msxsl:script implements-prefix="my-ext" language="C#">
public object evaluate(XPathNodeIterator context, string expression)
{
if (context.MoveNext()) {
return context.Current.Evaluate(expression);
}
else {
return null;
}
}
</msxsl:script>
<xsl:output method="xml" indent="yes"/>
<xsl:template match="#* | node()">
<xsl:copy>
<xsl:apply-templates select="#* | node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="xpath-test">
<xsl:copy>
<result>
<xsl:value-of select="my-ext:evaluate(., path)"/>
</result>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
I don't know whether Visual Studio allows you to enable that setting for its transformation menu.