C#学习笔记-协变与逆变

1，泛型接口和委托的逆变与协变
核心：
类型转换，参数只应用于引用类型

2，关键字：协变，out
Foo<父类>=Foo<子类>
Ioo<父类>=Ioo<子类>

3，关键字：逆变，in
Foo<子类>=Foo<父类>
Ioo<子类>=Ioo<父类>

返回值协变和参数逆变

    //泛型委托
    delegate T NormalEventHandler(T t);
    //delegate T CovarianceEventHandlerError(T tOut);//Error错误   1   方差无效: 类型参数“T”必须为“Test.CovarianceEventHandlerError.Invoke(T)”上有效的 逆变式。“T”为 协变。
    delegate T CovarianceEventHandler<out T>();
    //delegate T ContravarianceEventHandlerError(T tIn);//Error错误 1   方差无效: 类型参数“T”必须为“Test.ContravarianceEventHandlerError.Invoke(T)”上有效的 协变式。“T”为 逆变
    delegate void ContravarianceEventHandler<in T>(T tIn);
    delegate T2 ContravarianceEventHandler<in T1,out T2>(T1 tIn);
    //泛型接口
    interface INormal { };
    interface ICovariance<out T> { };
    interface IContravariance<out T> { };

    class ParentClass 
    {
        //基类,父类
        public void Fun()
        {
            MessageBox.Show("ParentClass");
        }
    }

    class ChildClass : ParentClass
    {
        //派生类,子类
        public void Fun()
        {
            MessageBox.Show("ChildClass");
        }
        public void Fun(int i)
        {
            MessageBox.Show("IIII--ChildClass");
        }
    }


    public partial class Form1 : Form
    {
        //协变
        CovarianceEventHandler CovarianceParent = null;
        CovarianceEventHandler CovarianceChild = null;
        //逆变
        ContravarianceEventHandler ContravarianceParent = null;
        ContravarianceEventHandler ContravarianceChild = null;

        ParentClass ParentFun() 
        {
            ParentClass p = new ParentClass();
            p.Fun();
            return p;
        }

        ChildClass ChildFun()
        {
            ChildClass c = new ChildClass();
            c.Fun(0);
            return c;
        }

        void ParentFun(ParentClass p)
        {
            p.Fun();
        }

        void ChildFun(ChildClass c)
        {
            c.Fun(0);
        }

        public Form1()
        {
            InitializeComponent();
            //协变-父=子
            CovarianceParent = ParentFun;
            CovarianceChild = ChildFun;
            CovarianceParent = CovarianceChild;
            //CovarianceChild = CovarianceParent;//Error,类型无法转换
            ParentClass obj = CovarianceParent();
            ChildClass obj1 = CovarianceChild();
            //ChildClass obj2 = CovarianceParent();//Error,类型无法转换
            if (obj is ChildClass && obj1 is ChildClass) 
            {
                MessageBox.Show("success");
            }
            if (obj is ParentClass && obj1 is ParentClass)
            {
                MessageBox.Show("fail");
            }
            obj.Fun();
            obj1.Fun();
            //obj2.Fun();
            //逆变-子=父
            ContravarianceParent = ParentFun;
            ContravarianceChild = ChildFun;
            ContravarianceChild = ContravarianceParent;
            //ContravarianceParent = ContravarianceChild;//Error,类型无法转换
            ContravarianceChild(new ChildClass());
            ContravarianceParent(new ParentClass());


        }