Roslyn

Roslyn, the Microsoft .NET compiler platform, is a set of open-source tools that provides compilers and code analysis APIs for C# and Visual Basic. It enables developers to analyze, modify, and generate code within editors, IDEs, and other tooling.

Roslyn exposes compiler services as libraries, allowing you to parse source code into syntax trees, inspect semantic information, and perform transformations. The platform includes the C# and Visual Basic compilers, as well as the code analysis APIs (such as the Syntax, SemanticModel, and Symbol APIs) and a scripting API for interactive code execution.

A key design goal is to enable compiler-as-a-service: tools can build custom analyzers, refactorings, code generators, and automated fixes without re-implementing language features. Roslyn also supports lightweight editing experiences and real-time diagnostics in editors.

In practice, Roslyn is used by Visual Studio to provide IntelliSense, syntax checking, and refactoring. It also underpins many third-party editors and CI tools that analyze or transform C# and VB code.

History and status: Microsoft announced Roslyn to create a shared, open-source compiler platform. The project was released to the public in 2014 and has since evolved with .NET Core and subsequent .NET releases.