数组并不满足OO的替换……（同样的东西还有struct等）
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        Son[] sons = new Son[5];
        Father[] fathers = sons;
        TestMethod(fathers);
    }
    private void TestMethod(Father[] ff)
    {
        ff[0] = new Daughter();//Here throw a exception.
    }

    private class Father { }
    private class Daughter : Father { }
    private class Son : Father { }