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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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