i2.cs

public interface IAnimal {
  void Eat();
  ...
  void Swim();
  void Fly();
}
public class Fish : IAnimal {
  public void Eat () {} 
  ...
  public void Swim() {}
  public void Fly() { throw new Exception("Cá éo biết bay"); }
}
public class Bird : IAnimal {
  public void Eat () {} 
  ...
  public void Swim() { throw new Exception("Chim éo biết bơi"); }
  public void Fly()
}