IValidatableObject interface?

Short answer: Use for cross-field validation within a model: public class Product : IValidatableObject {

Example code

public string Name { get; set; }
public decimal Price { get; set; }
public IEnumerable<ValidationResult> Validate(ValidationContext context) { if (Price < 0) { yield return new ValidationResult("Price must be positive"); }
}
}

Real-world example (ShopNest)

A ShopNest checkout request flows through middleware, hits a minimal API or controller, uses scoped services, and returns ProblemDetails on errors.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.