Onway

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

namespace ConsoleApplicationTest
{
    /*
     * 变体泛型
     *
     * .NET4，在泛型接口类型或者泛型委托类型里面，
     * 被关键字in声明的泛型参数，其类型可以从父类向子类逆变，只能用作输入参数
     * 被关键字out声明的泛型参数，其类型可以从子类向父类协变，只能用于返回值
     * 整个泛型类型是不限于用作输入参数还是作为返回值
     *
     * interface IType_IN_OUT<in T1, out T2>
     * IType_IN_OUT<Farmer, Person> farmerPersonType = null;
     * IType_IN_OUT<Person, Farmer> personfarmerType = null;
     * farmerPersonFinder = personFarmerFinder;
     *
     * 为什么不支持“变体”class呢？跟字段有关系么？
     */

    class Program
    {
        static void Main(string[] args)
        {
            IType_IN<Person> person_in = null;
            IType_IN<Farmer> farmer_in = null;
            farmer_in = person_in;
            Greeting(person_in);
            Console.WriteLine(GetTypeIn() == null);

            IType_OUT<Person> person_out = null;
            IType_OUT<Farmer> farmer_out = null;
            person_out = farmer_out;
            Greeting(farmer_out);
            Console.WriteLine(GetTypeOut() == null);

            Finder_IN<Person> personFinderIn = p => Console.WriteLine(p.GetType());
            Finder_IN<Farmer> farmerFinderIn = f => Console.WriteLine(f.GetType());
            farmerFinderIn = personFinderIn;
            Greeting(personFinderIn);
            Console.WriteLine(GetFarmerFinder().GetType());

            Finder_OUT<Person> personFinderOut = () => new Person();
            Finder_OUT<Farmer> farmerFinderOut = () => new Farmer();
            personFinderOut = farmerFinderOut;
            Greeting(personFinderOut);
            Console.WriteLine(GetPersonFinder().GetType());

            Finder_IN_OUT<Farmer, Person> farmerPersonFinder = f => new Person();
            Finder_IN_OUT<Person, Farmer> personFarmerFinder = p => new Farmer();
            farmerPersonFinder = personFarmerFinder;

            // interface IType_IN_OUT<in T1, out T2>
            IType_IN_OUT<Farmer, Person> farmerPersonType = null;
            IType_IN_OUT<Person, Farmer> personfarmerType = null;
            farmerPersonType = personfarmerType;

            Console.ReadKey();
        }

        static void Greeting(IType_OUT<Person> person)
        {
            Console.WriteLine(person == null);
        }

        static void Greeting(IType_IN<Farmer> farmer)
        {
            Console.WriteLine(farmer == null);
        }

        static IType_OUT<Person> GetTypeOut()
        {
            IType_OUT<Farmer> farmer = null;
            return farmer;
        }

        static IType_IN<Farmer> GetTypeIn()
        {
            IType_IN<Person> person_IN = null;
            return person_IN;
        }

        static void Greeting(Finder_OUT<Person> personFinder)
        {
            Console.WriteLine(personFinder().GetType());
        }

        static void Greeting(Finder_IN<Farmer> farmerFinder)
        {
            Console.WriteLine(farmerFinder.GetType());
        }

        static Finder_OUT<Person> GetPersonFinder()
        {
            Finder_OUT<Farmer> farmerFinder = () => new Farmer();
            return farmerFinder;
        }

        static Finder_IN<Farmer> GetFarmerFinder()
        {
            Finder_IN<Person> person = p => Console.WriteLine(p.GetType());
            return person;
        }
    }

    interface IType_OUT<out T>
    {
    }

    interface IType_IN<in T>
    {
    }

    interface IType_IN_OUT<in T1, out T2>
    {
    }

    delegate T Finder_OUT<out T>();

    delegate void Finder_IN<in T>(T t);

    delegate T2 Finder_IN_OUT<in T1, out T2>(T1 t1);

    class Person
    {
    }

    class Farmer : Person
    {
    }
}