Xsl3 1.3.1

Xsl3

An XSLT 3.0, XSLT 2.0 and XPath 3.1 processor for .NET, built on the engine behind System.Xml.Xsl.XslCompiledTransform: stylesheets are compiled to IL, and XSLT 1.0 stylesheets behave exactly as they do with stock .NET.

1.3.1: streaming verification checkpoint. XSLT 1.0 and 2.0 and XPath 3.1 are implemented. XSLT 3.0 works apart from supported streaming. Schema awareness remains opt-in through XsltSettings.SchemaAware.

Version 1.3.1 strengthens P7 verification of forward execution, Debug compiler checks, retained navigator memory, fork payloads and hidden grounding. Native initialization diagnostics now run in a separate workflow; the scheduled memory and throughput acceptance gates remain unchanged. Default execution stays tree-based; streaming remains experimental and unsupported.

Streaming remains experimental and supports-streaming remains no. Streamability = Strict controls static checking and does not enable forward execution by itself. Published default execution stays tree-based; forward execution requires the explicit development opt-in. The original streaming memory and throughput gates remain unchanged and unmet; this release does not complete full P7.

See the 1.3.1 release notes and checkpoint boundary. Strict analysis rejects XSLT 1.0 instructions in declared-streamable content with XTSE3430, including motionless instructions. Select XsltStreamabilityMode.Fallback when strict static checking is not intended. Keep Xsl3, Xsl3.Build and Xsl3.Cli at version 1.3.1 and regenerate saved stylesheets with matching packages.

Getting started

Xsl3 keeps XslCompiledTransform's API, so switching from System.Xml.Xsl is a using change:

using Xsl3;

var xslt = new XslCompiledTransform();
xslt.Load("style.xsl");
xslt.Transform("input.xml", "output.html");

For extra speed on XSLT 1.0, new in 0.4.0-alpha, set UseCompactTree before Load:

var xslt = new XslCompiledTransform { UseCompactTree = true };

The input is then parsed into Xsl3's compact tree, which takes about 60% of XPathDocument's memory. Transforms run about 1.5× faster than stock. Output is the same, except for the strings generate-id() returns and the order between nodes from different documents: compact trees come in the order they were built, so the input comes before the documents document() loads.

XsltArgumentList, XsltSettings, extension objects and parameters work as they do in System.Xml.Xsl. The TextWriter and Stream overloads of Transform serialize the way the stylesheet's xsl:output says, including XHTML, HTML5, JSON and the adaptive method.

Targets .NET 8 and .NET 10.

The XSLT 3.0 API

XsltCompiler, XsltExecutable and Xslt30Transformer start a stylesheet at a mode, a named template or a function, take and return XDM values (XdmNode, XdmAtomicValue, XdmMap, XdmArray), and give per-run handlers for messages and secondary results:

XsltExecutable executable = new XsltCompiler().Compile(new Uri("file:///styles/report.xsl"));
XdmValue rows = executable.CreateTransformer().CallTemplate(new QName("rows"));

Guides: getting started and moving from System.Xml.Xsl.

Conformance checked on 2026-10-29; Linux and Windows CI passed on .NET 8 and .NET 10. Performance figures below retain their original benchmark dates.

What's supported

XSLT 3.0 features include xsl:try, xsl:iterate, xsl:merge, accumulators, maps and arrays, text value templates, xsl:mode, static parameters and shadow attributes, packages (xsl:use-package, visibility, xsl:override), the JSON functions, and json and adaptive output.

Schema awareness is opt-in through XsltSettings.SchemaAware. Streaming is not yet a supported feature.

Secure by default

What a stylesheet can reach beyond its input is opt-in, through XsltSettings:

XsltSettings.TrustedXslt turns on the document functions, scripts and xsl:evaluate.

Performance

Xsl3 is faster than stock XslCompiledTransform on XSLT 1.0, with byte-identical output. Set UseCompactTree = true before Load to have a 1.0 stylesheet parse its input into Xsl3's compact tree. That tree takes about 60% of XPathDocument's memory. XSLT 2.0+ stylesheets always use it, for their input and for their temporary trees. Details: https://github.com/skuirrels/Xsl3/blob/main/docs/performance.md

Links

• Source, issues and roadmap: https://github.com/skuirrels/Xsl3
• Conformance details: https://github.com/skuirrels/Xsl3/blob/main/docs/conformance.md
• Differences from System.Xml.Xsl: https://github.com/skuirrels/Xsl3/blob/main/docs/divergences.md

MIT licensed. Xsl3 is an independent project and isn't affiliated with or endorsed by Microsoft or the .NET Foundation.