Convariance、Contravariance、Invariance

 1 class Animal
 2     {
 3         public int NumberOfLegs = 4;
 4     }
 5     class Dog : Animal
 6     {
 7        
 8     }
 9     class Program
10     {
11         static void Main()
12         {
13             Animal a1 = new Animal();
14             Animal a2 = new Dog();//将派生类的引用赋值给基类
15             Console.WriteLine("{0}",a2.NumberOfLegs);
16             Console.Read();
17         }
18     }

 class Animal 
    { 
        public int NumberOfLegs = 4;
    }
    class Dog : Animal { }
    class Program
    {
        delegate void Action<in T>(T a);//in 逆变关键字
        static void ActOnAnimal(Animal a)
        {
            Console.WriteLine(a.NumberOfLegs);
        }
        static void Main()
        {
            Action<Animal> act1 = ActOnAnimal;//创建方法参数为Animal类型的委托变量并赋值
            Action<Dog> dog1 = act1;//将变量赋值给方法参数为Dog类型的委托变量，其中Animal为Dog的基类！
            dog1(new Dog());
        }
    }