C# FluentValidation for a hierarchy of classes

public class Derived2Validator : AbstractValidator<Derived2>
{
    public Derived2Validator()
    {
        Include(new BaseValidator());
        Include(new Derived1Validator());
        RuleFor(d => d.Derived1Name).NotNull();
    }
}

Derived2Validator does not need to inherit BaseValidator or Derived1Validator.

Use the Include method to include the rules from other validators.