linq中的逆变和协变

案列：

 public class Animal { }
    public class Dog : Animal { }

    class Program
    {
        static void Main(string[] args)
        {
            Animal dog = new Dog();
            Dog dog1 = new Dog();


            List<Dog> dogs = new List<Dog>();
            //List<Animal> animals = dogs;
            List<Animal> animals = dogs.Select(d => (Animal)d).ToList();
            // out是协变，输出类型，作为结果返回
            // in是逆变，输入类型，作为参数
            // IEnumerable<out T> 这个T是输出类型，传入的是dog，可以协变为Animal。
            IEnumerable<Animal> animals1 = dogs;
            // Action<in T>(T obj);
            Action<Animal> actionAnimal = new Action<Animal>(a => { });
            Action<Dog> actionDog = actionAnimal;
            actionDog(dog1);


            Console.ReadKey();
        }