C# 协变 逆变

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

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

            Animal a1=new Animal();
            a1.Name="A1";
            Animal a2=new Animal();
            a2.Name="A2";

          

           //接口=OUT           
           // IEnumerable<Dog>
           // IEnumerator<Dog> 
           // IQueryable<Animal>
           // IGrouping<int,string>

            // 接口=IN
            //  IComparer<Dog>
            //  IComparable<Dog>
            //  IEqualityComparer<Dog>

            //  委托  IN | OUT       
            //  Action<int>
            //  Func<int,string>
            //  Converter<int,string>
            //  Comparison<int>
            //  Predicate<int>


            var listdog=GetDogs();
            IEnumerable<Animal> list1=listdog;
            foreach (var item in list1)
            {
                item.Call();
            }

            List<Animal> list2 = listdog.Select(d => (Animal)d).ToList();
            foreach (var item in list2)
            {
                item.Call();
            }
         
            

            //协变: 具体 赋给 抽象 
            IOut<Dog> myDogs = new TestOut<Dog>();
            IOut<Animal> myAnimals = myDogs;
            IOut<Animal> animals222=new TestOut<Dog>();

       

            //逆变: 抽象 赋给 具体 
            IIn<Animal> myAnimals2 = new TestIn<Animal>();            
            IIn<Dog> myDogs2 = myAnimals2;
            IIn<Dog> dogsss=new TestIn<Animal>();


            Console.ReadKey();
        }

        static IEnumerable<Animal> GetDogs()
        {
            Dog dog1=new Dog();
            dog1.Name="one";
            dog1.Age=11;
            Dog dog2=new Dog();
            dog2.Name="two";
            dog2.Age=22;

               DogEqual e=new DogEqual();
               e.Equals(dog1, dog2);
            


            List<Dog> listdog=new List<Dog>();
            listdog.Add(dog1);
            listdog.Add(dog2);
            return listdog;
        }

        static void ShowDog(IIn<Dog> dog)
        {
            
        }

        public interface IAnimal
        {
            void Call();
        }
        public  class Animal:IAnimal
        {
            public string Name;
            public virtual void Call()
            {
                Console.WriteLine("Animal");
            }
        }
        public class Dog : Animal
        {
            public new string Name;
            public int Age;
            public override void Call()
            {
                Console.WriteLine("Dog!");
            }
        }

        //out 协变  用于返回
        public interface IOut<out T>
        {
            T GetElement();
           
        }

        public class TestOut<T> : IOut<T> where T:new()
        {
            public T GetElement()
            {
                return new T();
            }
        }

        //in 逆变 用于传入
        public interface IIn<in T>
        {
            void ChangeT(T t);
        }
        public class TestIn<T> : IIn<T> where  T:IAnimal
        {
            public void ChangeT(T t)
            {
                t.Call();
            }
        }


        public class animalEqual : IEqualityComparer<Animal>
        {

            public bool Equals(Animal x, Animal y)
            {

                return x.Name.Equals(y.Name);
            }

            public int GetHashCode(Animal obj)
            {
                return 1;
            }

        }



        public class DogEqual : IEqualityComparer<Dog>
        {

            public bool Equals(Dog x, Dog y)
            {

                return x.Name.Equals(y.Name) && x.Age==y.Age;
            }

            public int GetHashCode(Dog obj)
            {
                return 1;
            }

        }
        
    }
}

 逆变：http://msdn.microsoft.com/zh-cn/library/ms173174%28VS.80%29.aspx