lamda表达式,扩展方法,委托练习(lamda表达式=委托)

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

namespace ConsoleApplication1
{
    public delegate T Function<out T>();    //泛型委托   out关键字:基于协变的变体
    //public delegate Strong Function2();     //普通委托
    public delegate void Operate<in T>(T instance);//泛型委托   in关键字:基于逆变的变体

    public class Program
    {
        static void Main(string[] args)
        {
            ////协变委托
            //Function<Strong> funStrong = new Function<Strong>(GetInstance);
            //Function<Weak> funWeak = funStrong;
            //Weak weak = funWeak();

            ////逆变委托
            //Operate<Weak> opWeak = new Operate<Weak>(DoSth);
            //Operate<Strong> opStrong = opWeak;
            //opStrong(new Strong());

            //Func<Students, Student> f = new Func<Students, Student>(GetStudent);

            Students ss = new Students(); ss.stus.Where(a => a.name == "");
            var c = ss.MySelect(a => a.MySelect2(b => b.name == "wushuangqi"));
            Console.WriteLine(c.id);

            Console.WriteLine(1.oper(2,  add));
            Console.WriteLine(2.oper(1, (a, b) => a - b));
        }

        static int add(int a,int b) {
            return a + b;
        }

        static Student GetStudent(Students ss)
        {
            return new Student();
        }

        static Strong GetInstance()
        {
            return new Strong();
        }

        static void DoSth(Weak weak)
        {
            //...
        }
    }

    /// <summary>
    /// 强类型
    /// </summary>
    class Weak
    {

    }

    /// <summary>
    /// 弱类型
    /// </summary>
    class Strong : Weak
    {

    }

    class Student
    {
        public string name { get; set; }
        public string id { get; set; }
    }

    class Students
    {
        public Students()
        {
            stus = new List<Student>();
            stus.Add(new Student { name = "wushuangqi", id = "1" });
            stus.Add(new Student { name = "wushuangqi", id = "2" });
            stus.Add(new Student { name = "lingyuan", id = "2" });
        }
        public List<Student> stus { get; set; }
    }



    static class MyHelp
    {
        public static Student MySelect(this Students ss, Func<List<Student>, Student> func)
        {
            //Student s = null;
            //func()
            return func(ss.stus);
            //return s;
            //return new Student();
        }

        public static Student MySelect2(this List<Student> sss, Func<Student, bool> func)
        {
            Student s = null;
            foreach (var item in sss)
            {
                if (func(item))
                {
                    s = item;
                    break;
                }
            }
            return s;
            //return new Student();
        }

        //public Student ddd(Students ss)
        //{
        //    foreach (Student s in ss.stus)
        //    {
        //        if (s.name.Equals("wushuangqi"))
        //        {
        //            return s;
        //        }
        //        else
        //        {
        //            return null;
        //        }
        //    }
        //    return null;
        //}
        public static int oper(this int a, int b, mydg dg)
        {
            return dg(a, b);
        }
    }

    public delegate int mydg(int a, int b);


}