显示转换(强制转换),访问子类中的值i

class Fruit { }

class Apple : Fruit

{

    public int i = 1;

}

private void button1_Click(object sender, EventArgs e)

{

    Fruit f = new Apple();

    Apple a = f as Apple //也可以写成 Apple a = (Apple)f; 显示转换,可以访问到Apple中的i

    MessageBox.Show("" + a.i);  //也可以将上二句简写 MessageBox.Show("" + ((Apple)f).i);

}