static void Main(string[] args)
{
//正常的匹配 返回父类 输入形参:无
HandlerMethod handler1 = FirstHandler;
handler1();
//协变 返回值Dog默认转换为Mammal(父类中装子类)
HandlerMethod handler2 = SecondHandler;
handler2();
//创建父类的对象
Manmal m = new Manmal();
//正常匹配 无返回值，输入的形参为父类
HandleModthod1 handler11 = ThirdHandler;
handler11(m);
//创建子类的对象
Dog d = new Dog();
//逆变，参数Dog默认可以转换成为Mammal
HandleModthod2 handle33 = ThirdHandler;
handle33(d);
}
public static void ThirdHandler(Manmal m)
{
Console.WriteLine("Third Handler");
}