The Metalama Compiler is a fork of Roslyn that introduces extension points enabling analyzer projects to execute arbitrary transformations via the ISourceTransformer interface.

The Metalama Compiler is actively and professionally maintained by PostSharp Technologies.

This repository is a part of Metalama, a high-level meta-programming framework for C#. The first pillar of Metalama, boilerplate reduction, relies on the Metalama Compiler to implement aspect-oriented programming. Metalama utilizes several other Roslyn extension points: analyzers, diagnostic suppressors, source generators, code fix providers, and code refactoring providers. However, no extension point was previously available for transforming source code during compilation, hence the reliance on this fork.

For a detailed comparison of the modifications in the Metalama Compiler compared to the original Roslyn, please refer to this article.

Features

The Metalama Compiler introduces the following features to Roslyn:

• Source transformers. This feature allows analyzer projects to execute arbitrary transformations of source code during compilation via the ISourceTransformer interface.

• Ordering. Multiple source transformers can be ordered using the TransformerOrderAttribute assembly-level custom attribute or the MetalamaCompilerTransformerOrder MSBuild property (see below).

  [!Warning] Overloading a project with too many source transformers can lead to performance issues.

  Analyzers can be ordered before or after transformers using the MetalamaSourceOnlyAnalyzers property (see below).

• Source mapping. Diagnostics and PDBs are mapped to the source code by default. This behavior can be altered using one of the new MSBuild properties defined by the Metalama Compiler (see below). This is arguably the most complex feature of the Metalama Compiler.

• Managed resources. The TransformerContext argument of ISourceTransformer.Execute provides read-write access to managed resources, enabling the addition of resources to the reference assembly.

Building

To build the Metalama Compiler, clone the repository and execute the following command from the command line:

.\Build.ps1 build

Documentation

Getting started

Step 1. Building a source transformer

To create a source transformer:

1. Establish a new .NET Standard 2.0 class library project and add a reference to the Metalama.Compiler.Sdk package.
2. Create a new class that implements the ISourceTransformer interface.
3. Add the [Transformer] custom attribute to this class.
4. Implement the Execute method. This method receives a TransformerContext object. Use this object to inspect the current compilation, add syntax trees, modify syntax trees, report diagnostics, or suppress diagnostics.

using Metalama.Compiler;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;

namespace Metalama.SourceTransformer
{
    [Transformer]
    internal class SourceTransformer : ISourceTransformer
    {
        public void Execute( TransformerContext context )
        {

        }
    }
}

Step 2. Packaging a source transformer

The packaging of your transformer should adhere to the same rules as analyzers and source generators. That is, your output dll should be in the analyzers/dotnet/cs folder, not under lib.

Follow this example from the Roslyn documentation:

1. Define the IncludeBuildOutput=False property to prevent the project output from being added to the lib directory of the package.

2. Use the following to include the output under the analyzers/dotnet/cs:

   <ItemGroup>
    <None Include="$(OutputPath)\$(AssemblyName).dll"
          Pack="true"
          PackagePath="analyzers/dotnet/cs"
          Visible="false" />
   </ItemGroup>

Step 3. Using a source transformer

In any project that uses the source transformer:

• Add the package containing your transformer.

  If you need a ProjectReference instead of a PackageReference, use the following code:

  <ItemGroup>
      <ProjectReference Include="..\PathTo\SourceGenerator.csproj"
                        OutputItemType="Analyzer"
                        ReferenceOutputAssembly="false" />
  </ItemGroup>
  

• Add the Metalama.Compiler package. You can specify PrivateAssets="all" if you don't want the package to flow to the projects referencing this project.

API Documentation

See Metalama.Compiler

Architecture

The public API of the extensions introduced by the Metalama Compiler is included in the Metalama.Compiler.Sdk package. This package contains only interfaces and stubs.

The Metalama.Compiler package replaces the Roslyn compiler that comes with Visual Studio or the .NET SDK, but only for projects that reference the package. Metalama.Compiler is actually a fork of Microsoft.Net.Compilers.Toolset. It contains the implementation of Metalama.Compiler.Sdk.

MSBuild properties

The Metalama Compiler can be configured using several custom MSBuild properties from the csproj file of a user project:

• MetalamaEmitCompilerTransformedFiles: Set this property to true to write transformed files to the obj/Debug/metalama or obj/Release/metalama directory. The default is true if MetalamaDebugTransformedCode is enabled and false otherwise.
• MetalamaCompilerTransformedFilesOutputPath: This property can be used to set the directory where transformed files are written instead of obj/Debug.
• MetalamaCompilerTransformerOrder: This property is a semicolon-separated list of namespace-qualified names of transformers. It is necessary for setting the execution order of transformers if the order has not been fully specified by the transformers using [TransformerOrder].
• MetalamaDebugTransformedCode: Set this property to true to produce diagnostics and PDB sequence points in transformed code. Otherwise, locations are attempted to be mapped to the original user code. The default is false.
• MetalamaDebugCompiler: Set this property to true to trigger Debugger.Launch().
• MetalamaSourceOnlyAnalyzers contains the list of analyzers that must execute on the source code instead of the transformed code. This is a comma-separated list that can include the assembly name, an exact namespace (namespace inheritance rules do not apply), or the exact full name of an analyzer type.

Note

If MetalamaDebugTransformedCode is set to true, but MetalamaEmitCompilerTransformedFiles is explicitly set to false (and no custom CompilerTransformedFilesOutputPath is provided), then transformed sources should be used for debugging and diagnostics, but cannot be written to disk.

For debugging, this means transformed sources are embedded into the PDB. For diagnostics, this means the reported locations are nonsensical and the user is warned about this.