c#泛型參數 協變和逆變

能夠參考 https://www.cnblogs.com/qixuejia/p/4383068.html 個人測試代碼以下：

public class Animal { }
    public class Dog : Animal { }

    public class testclass
    {
        public int i = 1;
    }
 //協變和 逆變
            Func<Object,bool> parent = (p)=>{Console.WriteLine(p.GetType().FullName.ToString());return true;};
            Func<testclass, bool> child = parent;

            child(new testclass());

            //Func<testclass, bool> chx = (cp) => { Console.WriteLine(cp.GetType().FullName.ToString()); return true; };
            ////Func<Object, bool> phx = chx;  //此句有問題
            //Func<Object, bool> phx = (Func<Object, bool>)chx; //換成這句就編譯成功,但運行時出錯。
            //phx(new testclass());

            Func<bool, testclass> ch = (b) => { var t = new testclass(); Console.WriteLine("t.i=" + t.i); return t; };
            Func<Boolean, Object> ph = ch;
            ph(false);


            List<Dog> lsdog = new List<Dog>();
            //List<Animal> lsanimal = lsdog;//沒法轉換
            List<Animal> lsanimal = lsdog.Select(d => { return (Animal)d; }).ToList();//能夠正常轉換，但這種作法太麻煩

            List<Animal> lsanimal1 = new List<Animal>();
            //List<Dog> lsdog1 = lsanimal1; //也沒法轉換

            //List<T>中沒有用到in和out關鍵字，因此List<>沒有協變和逆變功能。用IEnumerable<out T>來看看
            List<Animal> lsanimal2 = new List<Animal>();
            IEnumerable<Animal> ieanimal = lsanimal2; //子類轉換成父接口固然成功（只不過是相同的泛型而已）
            //IEnumerable<Dog> iedog = ieanimal;//不成功 爲何不成功呢？由於IEnumerable<out t>中用的是out關鍵字，表明之後轉換時，只能進行輸出轉換，既然
            //是輸出轉換，若是是子轉換成父（協變）就能夠成功，若是是父轉子（逆變）就不能成功。

            List<Dog> lsdog2 = new List<Dog>();
            IEnumerable<Dog> iedog = lsdog2;//也是相同泛型參數的子類轉換成父接口，固然成功
            IEnumerable<Animal> ieanimal2 = iedog; //協變，泛型參數子轉換成父。成功


            //out關鍵字指出能夠協變，in指出能夠逆變
            //c#中IEnumerable<out T>,IEnumerator<out T>,IQueryable<out T>都是能夠協變的
            //IComparer<in T>,ICompareable<in T>是能夠逆變的