C# 9 Source Generators: Removing Reflection

Reflection is slow. It happens at runtime, bypasses type safety, and prevents trimming. Source Generators solve this by generating code at compile time. In this guide, we build a generator that automatically implements a `MapTo` method for DTOs, replacing AutoMapper.

The Goal

[AutoMap(typeof(UserEntity))]
public partial class UserDto
{
    public string Username { get; set; }
}

// Generated Code should look like:
partial class UserDto
{
    public void MapFrom(UserEntity entity)
    {
        this.Username = entity.Username;
    }
}

The Generator Logic

[Generator]
public class MapperGenerator : ISourceGenerator
{
    public void Execute(GeneratorExecutionContext context)
    {
        var syntaxReceiver = (MySyntaxReceiver)context.SyntaxReceiver;
        foreach (var classDecl in syntaxReceiver.CandidateClasses)
        {
            // Analyze properties using Roslyn Semantic Model
            // Generate StringBuilder content
            context.AddSource($"{className}_Map.g.cs", sourceText);
        }
    }
}

Key Takeaways

• Source Generators enable **Zero-Overhead abstractions**.
• They are essential for **Native AOT** compatibility.
• Debug them by setting `<IsRoslynComponent>true</IsRoslynComponent>`.