Class-----继承理解

public class A
...{
    public virtual void play()
    ...{
        System.Console.WriteLine("a我在打球");
    }
}
public class B:A
...{
    public override void play()
    ...{
        System.Console.WriteLine("b我在打球");
    }
    public void say()
    ...{
        System.Console.WriteLine("b说说你好你好啊啊 ");
    }
}
/**//********************************************************************/
                                          A a=new A();
        a.play();
        B b=new B();
        b.play();
        b.say();
        A x=new B();
        x.play();//输出 b在打球    
    //    x.say(); 错误的提示 没有该方法
    /**//*
        error tip: 需要强制转换
        B y=new A();
        y.play();            
        */