Mono.TextTemplating

NOTE: To use a template at runtime in your app, you do not need to host the engine. It is generally preferable to use dotnet-t4 to generate a runtime template class and compile that into your app, as this has substantially less overhead than hosting the engine.

Mono.TextTemplating is an open-source reimplementation of the Visual Studio T4 text templating engine, and supports C# 10 and .NET 6. This package is the engine package, which can be used to host the T4 engine in your own app.

By default the engine uses the C# compiler from the .NET SDK, but the Mono.TextTemplating.Roslyn package can be used to bundle a copy of the Roslyn C# compiler and host it in-process. This may improve template compilation performance when compiling multiple templates, and guarantees a specific version of the compiler.

Usage

This will read a template from templateFile, compile and process it, and write the output to outputFile:

var generator = new TemplateGenerator ();
bool success = await generator.ProcessTemplateAsync (templateFilename, outputFilename);

This does the same thing as a series of lower-level steps, allowing it to provide additional compiler arguments by modifying the TemplateSettings:

string templateContent = File.ReadAllText (templateFilename);

var generator = new TemplateGenerator ();
ParsedTemplate parsed = generator.ParseTemplate (templateFilename, templateContent);
TemplateSettings settings = TemplatingEngine.GetSettings (generator, parsed);

settings.CompilerOptions = "-nullable:enable";

(string generatedFilename, string generatedContent) = await generator.ProcessTemplateAsync (
    parsed, inputFilename, inputContent, outputFilename, settings
);

File.WriteAllText (generatedFilename, generatedContent);

API Overview

Hosting

In most cases, you need only use or subclass TemplateGenerator:

• It implements ITextTemplatingEngineHost and ITextTemplatingSessionHost with a default implementation that can be overridden if needed.
• It wraps a TemplateEngine instance and provides simplified ProcessTemplateAsync() and PreprocessTemplateAsync() methods.

VS T4 Compatibility

Mono.TextTemplating has session, host and directive processor interfaces and classes in the Microsoft.VisualStudio.TextTemplating namespace that are near-identical to the original Visual Studio T4 implementation. This allows older T4 templates and directive processors to work with Mono.TextTemplating with few (if any) changes.

• ITextTemplatingEngineHost
• ITextTemplatingSessionHost, ITextTemplatingSession, TextTemplatingSession
• IDirectiveProcessor, IRecognizeHostSpecific, DirectiveProcessor, DirectiveProcessorException, RequiresProvidesDirectiveProcessor, ParameterDirectiveProcessor

The Microsoft.VisualStudio.TextTemplating.(ITextTemplatingEngine,Engine) hosting API is supported but deprecated.

Advanced

For advanced use, some lower level classes and methods are accessible:

• TemplatingEngine: generates C# classes from T4 templates and compiles them into assemblies
• TemplateGenerator.ParseTemplate(): uses a Tokenizer to parse a template string into a ParsedTemplate
• Tokenizer: tokenizes an T4 input stream
• ParsedTemplate: provides direct access to the segments and directives of a parsed template
• TemplatingEngine.GetSettings(): uses the directives in a ParsedTemplate to initialize a TemplateSettings
• TemplateSettings: settings that control code generation and compilation.
• CompiledTemplate: a template that has been compiled but not executed

Differences from VS T4

The Mono.TextTemplating engine contains many improvements over the original Visual Studio T4 implementation, including:

• It supports the latest .NET APIs and C# language version
• The engine and the code it generates are compatible with .NET Core and .NET 5+
• It executes templates in an AssemblyLoadContext, which allows the generated assemblies to be garbage collected (where supported)
• Parameter directives may use primitive types: <#@ parameter name="Foo" type="int" #>