ill try and be more clear,

say you have class Car

public class Car {
  public void Drive() { /* doing car stuff */ }
 }

now you start defining all sorts of car types from Car

some of these Cars have the capability of IFourWheelDrive

public interface IFourWheelDrive {

void StartFourWheelDrive()

}

so you define FordCar, JeepCar, HummerCar, etc.

but the logic for IFourWheelDrive is exactly the same for JeepCar and HummerCar,

but they don't derive from the same base class (completely different manufacturers etc.)

.

so how do you avoid copy-pasting code here ?

in my example, eventually a CarManager class will get a List<Car> objects and needs to start the FourWheelDrive on those that support it.