What is the Roslyn compiler API?

Roslyn is Microsoft's open-source C# and VB.NET compiler platform — it's a "compiler as a service," exposing rich APIs for analyzing and generating code. Key APIs: Syntax API: parse C# code into a syntax tree (SyntaxTree, SyntaxNode) and analyze its structure. Semantic API: understand the meaning of code — type information, symbol resolution, data flow. Workspace API: open and modify Visual Studio solutions and projects programmatically. Use cases: (1) Analyzers: custom code quality rules that show warnings/errors in the IDE and CI. (2) Code fixes: automatic fixes for analyzer findings. (3) Source generators: generate code at compile time based on existing code. (4) Refactoring tools: IDE extensions. (5) Scripting: evaluate C# scripts at runtime. Roslyn analyzers are distributed as NuGet packages and integrate seamlessly into the build pipeline. Tools like Roslynator, StyleCop, and many framework-specific analyzers are built on Roslyn.