01_Facetting.md

What is Facetting?

Facetting is the process of defining focused views of a larger model at compile time.

Instead of manually writing separate DTOs, mappers, and projections, Facet allows you to declare what you want to keep — and generates everything else.

You can think of it like carving out a specific facet of a gem:

• The part you care about
• Leaving the rest behind.

Why Facetting?

• Reduce duplication across DTOs, projections, and ViewModels
• Maintain strong typing with no runtime cost
• Stay DRY (Don't Repeat Yourself) without sacrificing performance
• Works seamlessly with LINQ providers like Entity Framework

Example

Source model:

public class User
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }
}

Define a facet:

[Facet(typeof(User), exclude: nameof(User.Email))]
public partial class UserDto { }

You get:

• A mapped constructor
• A LINQ Expression Projection
• A partial class or record ready to extend