Source Generators in C# 9: Compile-Time Code Generation

Source Generators allow you to inspect code during compilation and generate new C# files on the fly. Unlike Reflection (runtime slow) or IL Weaving (complex), Source Generators are fast, debuggable, and integrated into Roslyn.

How it Works

flowchart LR
    Source[User Code] --> Compiler[C# Compiler]
    Compiler -->|Analyze| Generator[Source Generator]
    Generator -->|Emit .cs| NewCode[Generated Code]
    NewCode --> Compiler
    Compiler --> Output[DLL/EXE]
    
    style Generator fill:#E1F5FE,stroke:#0277BD

Example: INotifyPropertyChanged

Automate the boilerplate of MVVM.

[Generator]
public class NotifyGenerator : ISourceGenerator
{
    public void Execute(GeneratorExecutionContext context)
    {
        // Find classes with [AutoNotify] attribute
        // Generate partial class with boilerplace code
    }
    
    public void Initialize(GeneratorInitializationContext context) {}
}

Key Takeaways

• Generators can **add** code, but cannot **modify** existing code.
• Use `partial` methods and classes to create hook points.
• Expect massive performance improvements for serializers (System.Text.Json) and mappers.