System.PlatformNotSupported Exception in Dot Net Core XML load and transform with XSLT having VB script

Note: This answer is taken from the top-level comments on this question and this github repo (which is linked in the comments). I just pulled everything together in an answer to make it easier for future readers.

You can use extension objects to call your own C# methods from XSLT.

For example, define the methods you want to use:

public class XsltFunctions
{
    public static string ToLower(XPathNodeIterator iterator)
    {
        return iterator.OfType<XPathNavigator>().FirstOrDefault()?.Value.ToLower();
    }
}

Then register that class as an extensions object when running your XSLT. (Pay attention to the AddExtensionObject method.)

public string RunXslt(string inputXml)
{
    // xml transform pointing to custom code
    var xsltDocumentString = $@"<xsl:stylesheet version=""1.0"" xmlns:xsl=""http://www.w3.org/1999/XSL/Transform"" xmlns:msxsl=""urn:schemas-microsoft-com:xslt"" xmlns:user=""{nameof(XsltFunctions)}""> 
    <xsl:template match=""MyElt"">
<MyElt>
    <OriginalValue>
        <xsl:copy-of select=""node()""/>
    </OriginalValue>
    <ToLowerValue>
        <xsl:value-of select=""user:ToLower(node())""/>
    </ToLowerValue>
</MyElt>
</xsl:template>
</xsl:stylesheet>";

    // Compile the style sheet.
    var xslt_settings = new XsltSettings();
    var xslt = new XslCompiledTransform();
    var styleSheetReader = new XmlTextReader(new StringReader(xsltDocumentString));
    xslt.Load(styleSheetReader, xslt_settings, new XmlUrlResolver());

    // Add capability to ref external functions
    var xsltArgList = new XsltArgumentList();
    xsltArgList.AddExtensionObject(nameof(XsltFunctions), new XsltFunctions());

    // Load the XML source file.
    XmlReader inputDataReader = new XmlTextReader(new StringReader(inputXml));

    // Create an XmlWriter.
    var settings = new XmlWriterSettings();
    settings.OmitXmlDeclaration = true;
    settings.Indent = true;

    var output = new StringBuilder();
    var writer = new XmlTextWriter(new StringWriter(output));

    // Execute the transformation.
    xslt.Transform(inputDataReader, xsltArgList, writer);

    // return the transformed XML response to the caller
    return output.ToString();
}

Tested in .NET Core 2.1 and .NET 5, using the following NUnit test.

[Test]
public void RunXsltWithExternalFunctions()
{
    // Arrange.
    var inputXml = @"<?xml version=""1.0"" encoding=""utf-8""?>
<MyXml>
    <MyElt>
        MyEltValue
    </MyElt>
</MyXml>";

    // Act.
    var outputXml = RunXslt(inputXml);

    // Assert.
    Assert.That(outputXml, Does.Contain("myeltvalue"));
}

Source

See also: XSLT Extension Objects