C#(.NET Core) 泛型<T>中协变(covariant)和逆变(contravariant)的使用

1､泛型中协变(covariant)

简单来说，协变用法是只能放在接口或者委托的泛型参数前面，是用来修饰返回值。具体使用如下代码：

    /// <summary>
    /// out 协变 只能是返回结果
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public interface ICustomerListOut<out T>
    {
        T Get();
        //void Show(T t);
    }
    public class CustomerListOut<T> : ICustomerListOut<T>
    {
        public T Get()
        {
            return default(T);
        }
        //public void Show(T t)
        //{
        //}
    }

2､泛型中逆变(contravariant)

也是只能放在接口或者委托的泛型参数前面，但是用来修饰传入参数。具体使用如下代码：

 public interface IMyList<in inT, out outT>
    {
        void Show(inT t);
        outT Get();
        outT Do(inT t);
        ////out 只能是返回值   in只能是参数
        //void Show1(outT t);
        //inT Get1();
    }

3､协变和逆变的使用

协变和逆变都是用在接口或者委托的泛型参数前面，一个用来修饰返回值，一个是修饰传入参数，使用示例代码如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Cjavapy.Extend
{
    /// <summary>
    /// .net4.0
    /// 只能放在接口或者委托的泛型参数前面
    /// out 协变covariant    修饰返回值 
    /// in  逆变contravariant  修饰传入参数
    /// </summary>
    public class CCTest
    {
        public static void Show()
        {
            {
                Bird bird1 = new Bird();
                Bird bird2 = new Sparrow();
                Sparrow sparrow1 = new Sparrow();
                //Sparrow sparrow2 = new Bird();
            }
            {
                List<Bird> birdList1 = new List<Bird>();
                //List<Bird> birdList2 = new List<Sparrow>();
                //应该可以呀   一堆麻雀当然是一堆鸟
                //没有父子关系
                List<Bird> birdList3 = new List<Sparrow>().Select(c => (Bird)c).ToList();
            }
            {//协变
                IEnumerable<Bird> birdList1 = new List<Bird>();
                IEnumerable<Bird> birdList2 = new List<Sparrow>();
                Func<Bird> func = new Func<Sparrow>(() => null);
                ICustomerListOut<Bird> customerList1 = new CustomerListOut<Bird>();
                ICustomerListOut<Bird> customerList2 = new CustomerListOut<Sparrow>();
            }


            {//逆变
                ICustomerListIn<Sparrow> customerList2 = new CustomerListIn<Sparrow>();
                ICustomerListIn<Sparrow> customerList1 = new CustomerListIn<Bird>();
                ICustomerListIn<Bird> birdList1 = new CustomerListIn<Bird>();
                birdList1.Show(new Sparrow());
                birdList1.Show(new Bird());
                Action<Sparrow> act = new Action<Bird>((Bird i) => { });
            }

            {
                IMyList<Sparrow, Bird> myList1 = new MyList<Sparrow, Bird>();
                IMyList<Sparrow, Bird> myList2 = new MyList<Sparrow, Sparrow>();//协变
                IMyList<Sparrow, Bird> myList3 = new MyList<Bird, Bird>();//逆变
                IMyList<Sparrow, Bird> myList4 = new MyList<Bird, Sparrow>();//逆变+协变
            }
        }
    }
    public class Bird
    {
        public int Id { get; set; }
    }
    public class Sparrow : Bird
    {
        public string Name { get; set; }
    }
    public interface ICustomerListIn<in T>
    {
        //T Get();
        void Show(T t);
    }
    public class CustomerListIn<T> : ICustomerListIn<T>
    {
        //public T Get()
        //{
        //    return default(T);
        //}
        public void Show(T t)
        {
        }
    }
    /// <summary>
    /// out 协变 只能是返回结果
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public interface ICustomerListOut<out T>
    {
        T Get();
        //void Show(T t);
    }
    public class CustomerListOut<T> : ICustomerListOut<T>
    {
        public T Get()
        {
            return default(T);
        }
        //public void Show(T t)
        //{
        //}
    }




    public interface IMyList<in inT, out outT>
    {
        void Show(inT t);
        outT Get();
        outT Do(inT t);
        ////out 只能是返回值   in只能是参数
        //void Show1(outT t);
        //inT Get1();
    }
    public class MyList<T1, T2> : IMyList<T1, T2>
    {
        public void Show(T1 t)
        {
            Console.WriteLine(t.GetType().Name);
        }
        public T2 Get()
        {
            Console.WriteLine(typeof(T2).Name);
            return default(T2);
        }
        public T2 Do(T1 t)
        {
            Console.WriteLine(t.GetType().Name);
            Console.WriteLine(typeof(T2).Name);
            return default(T2);
        }
    }
}

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