Defines the interface between the transformed compile-time user code and the Metalama engine. The package should normally never be referenced in any user project.

PostSharp, Metalama, AOP, csharp, dotnet
Install-Package Metalama.Framework.CompileTimeContracts -Version 0.5.63-preview




You can try Metalama in your browser, without installing anything, at

This is the home repo for Metalama, a Roslyn-based meta-programming framework C# with focus on aspect-oriented programmning, code generation and code validation. We use this repo for discussions and bug reports.

Table of contents


Metalama is an extension of the Microsoft "Roslyn" C# compiler that allows you to automatically transform your source code at build time or design time based on encapsulated code transformations called aspects. Metalama can be used for aspect-oriented programming, code generation, or architecture validation.

Metalama is intended to replace the MSIL-based stack that is now the foundation of PostSharp.

Documentation and Examples

Link Description
Documentation Conceptual and API documentation.
Try Metalama Try Metalama from your browser. Based on
Metalama.Samples A dozen of examples in a GitHub repo.
Metalama.Open.AutoCancellationToken A low-level Metalama aspect that adds cancellation tokens to your method declarations and your method calls.
Metalama.Open.Virtuosity A low-level Metalama aspect that makes your methods virtual. (A fork of Virtuosity.Fody.)
Metalama.Open.Costura A low-level Metalama aspect that embeds dependent assemblies as managed resources. (A fork of Costura.Fody.)
Metalama.Framework.Extensions Open-source extensions to Metalama.Framework.

Examples and code snippets are open-source and tested:

Documentation Snippets Metalama.Samples


Metalama is currently released under the terms of the Evaluation License of PostSharp.

Metalama is currently in EARLY PREVIEW stage and is not intended for commercial use. Any version of the Metalama preview will stop working 90 days after it has been built. To continue using it, you will need to update to a newer preview.

We would love your feedback

If you have any feedback regarding Metalama, please open an issue, start a discussion, or contact us directly at

How does Metalama compare to Roslyn source generators?

Unlike Roslyn source generators, Metalama:

  • can replace or enhance hand-written code (Roslyn source generators are additive only: you just can add partial classes);
  • allows you to write aspects (or code transformations):
    • in your main project (instead of a separate project),
    • using the C# language, with Intellisense and code validation (instead of building a string);
  • is therefore a real and complete framework for aspect-oriented programming in C#, with the same level of functionality than what exists in other languages (such as AspectJ for Java).

How does Metalama compare to PostSharp MSIL?

Metalama was designed from scratch. It is based on best lessons learned from PostSharp MSIL during the last 15 years, and addresses the main obstacles that are now hindering PostSharp MSIL.

You will enjoy the following benefits with Metalama compared to PostSharp:

  • Faster builds: Metalama runs directly inside the compiler process (it is a fork of Roslyn), does not require an external process, and is therefore much faster;
  • More powerful transformations: The templating technology used by Metalama allows for more control over code than what was possible with PostSharp MSIL.
  • Better multi-platform support: Metalama does not load the whole project being built in the compiler process, therefore it avoids the cross-compilation issues that have plagued PostSharp for many years;
  • Better design-time experience: You will see introduced members and interfaces in Intellisense because Metalama will do that at design time and not at post-compilation time. No need for weird casts.
  • Better run-time performance: Because of code generation improvements, you can create aspects that execute much faster.
  • Better debugging experience: You can switch from source code view to transformed code view can debug the code exactly how it is executed.

Compatibility with PostSharp MSIL

How compatible do we intend to be with PostSharp MSIL? How much code will you need to rewrite?

It has been 12 years since the last major breaking change in PostSharp. Do you remember the .NET landscape in 2008? Clearly, we cannot build a new platform by keeping compatibility with designs that were optimal 12 years ago. However, we understand that PostSharp is used by thousands and we want to find a compromise between modernity and backward compatibility.

We have already taken the following compromise:

  • your aspect code (typically less than a dozen of classes) will need to be totally rewritten,
  • your business code should not be affected.