New Post: RuleFor Object

Replying to myself, in order someone there is in same situation
The key is to cast object to right type.

public class ClassAValidator : AbstractValidator<ClassA>
{
public ClassAValidator()

{

   When(c => c.ItemSelector == ItemSelectorEnum.TypeA), () =>
      {
         RuleFor(c => c.Item as TypeAClass).SetValidator(new TypeAClassValidator());
      }

}
}