A C# source generator that auto-generates DTO mapping code from [Facette]-annotated partial records — zero reflection, zero runtime dependencies.

GitHub Repository

This project is entirely vibe coded with Claude Code.

Introduction

Facette is a C# source generator that automatically creates DTO mapping code from annotated partial records. Instead of writing tedious FromSource, ToSource, and LINQ projection methods by hand — or relying on runtime reflection — Facette generates all of it at compile time.

Why Facette?

• Zero reflection — all mapping code is generated at compile time
• Zero runtime dependencies — the Facette.Abstractions package contains only attributes
• Full IntelliSense — generated code is visible in your IDE
• LINQ projection support — generates Expression<Func<TSource, TDto>> for efficient database queries
• Compile-time diagnostics — catches configuration mistakes before you run the app

How it works

Facette uses the Roslyn incremental source generator API. When you annotate a partial record with [Facette(typeof(SourceType))], the generator inspects the source type at compile time and emits:

1. FromSource(source) — a static factory that maps source → DTO
2. ToSource() — an instance method that maps DTO → source
3. Projection — a static Expression<Func<TSource, TDto>> for LINQ/EF Core queries
4. Mapper extensions — ToDto(), ProjectToDto() extension methods on the source type

What you write

[Facette(typeof(Employee), "SocialSecurityNumber")]
public partial record EmployeeDto;

What gets generated

public partial record EmployeeDto
{
    public int Id { get; init; }
    public string FirstName { get; init; }
    public string LastName { get; init; }
    // ... all properties except SocialSecurityNumber

    public static EmployeeDto FromSource(Employee source) { /* ... */ }
    public Employee ToSource() { /* ... */ }
    public static Expression<Func<Employee, EmployeeDto>> Projection => /* ... */;
}

No boilerplate. No runtime overhead. Just annotate and go.

Credits

Facette was heavily inspired by:

• Facet — a source generator that generates DTOs, mappings, and LINQ projections from domain models, with a similar attribute-driven approach
• Mapperly — a compile-time object mapper for .NET that pioneered the source-generator approach to mapping

This project is entirely vibe coded with Claude Code.