public interface IBall
{
    void Play();
}

public class Football : IBall
{
    public void Play()
    {
      Console.WriteLine("我喜欢足球");
    }
}
public class Basketball : IBall
{
    public void Play()
    {
      Console.WriteLine("我喜欢篮球");
    }
}
public class Volleyball : IBall
{
    public void Play()
    {
      Console.WriteLine("我喜欢排球");
    }
}

public class SportsMan
{
    private IBall ball;
    public void SetHobby(IBall myBall)
    {
      ball = myBall;
    }
    public void StartPlay()
    {
      ball.Play();
    }
}

class Program
{
    static void Main(string[] args)
    {
      IBall ball = null;
      SportsMan man = new SportsMan();
      while (true)
      {
        Console.WriteLine("选择你喜欢的球类项目(1=足球， 2=篮球，3=排球)");
        string input = Console.ReadLine();
        switch (input)
        {
          case "1":
            ball = new Football();
            break;
          case "2":
            ball = new Basketball();
            break;
          case "3":
            ball = new Volleyball();
            break;
        }
        man.SetHobby(ball);
        man.StartPlay();
      }
    }
}