class Animal    { }    class Dog : Animal    { }    class Program    {        static void Main(string[] args)        {            //返回类型协变            Func<Animal, Animal> s = doThing;            //参数类型逆变            Action<Dog> s2 = doThing2;        }                static Dog doThing(Animal x)        {            return null;        }        static void doThing2(Animal x)        {        }    }