Custom Validation (IValidatableObject, Custom Validators)?

Short answer: IValidatableObject: public IEnumerable<ValidationResult> Validate(ValidationContext context) Custom Attribute: public class MyCustomAttribute : ValidationAttribute {

Example code

public override bool IsValid(object value) { ... }
}

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.