linq to object

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

namespace Linq
{
    class LinqToObject
    {
        private List<Student> listStu = null;
        private List<ClassStu> listClass = null;
        private Random random = new Random();
        public void SetList()
        {
            
            listStu = new List<Student>();
            listClass = new List<ClassStu>();
            for (int i = 0; i < 30; i++)
            {
                var student = new Student()
                {
                    id=random.Next(1,30),
                    name=$"阿毛{random.Next(0,30)}",
                    age = random.Next(10, 40)
                };
                var classStu = new ClassStu()
                {
                    id = random.Next(1, 30),
                    name = $"班级{random.Next(1, 20)}",
                    number = random.Next(20, 50)
                };

                listStu.Add(student);
                listClass.Add(classStu);
            }
        }

        public void Show()
        {
            SetList();
            listStu.Where<Student>(s => s.age < 10).ToList().ForEach((m) => Console.WriteLine($"该学生的年龄小于10的有{m.name}==>{m.age}"));
        }

        //延迟加载
        public void DelayShow()
        {
            SetList();
            {
                Console.WriteLine("**************************************");
                //查询运算符
                var list = Enumerable.Where(listStu, (s) =>
                {
                    Console.WriteLine("123"); //在返回结果之前添加一些别的可以延迟
                    return s.age < 10;
                });
                foreach (var item in list)
                {
                    Console.WriteLine($"该学生的年龄小于10的有{item.name}==>{item.age}");
                }
            }
            
            {
                //投影第1种
                Console.WriteLine("**************************************");
                //查询运算符
                var list = Enumerable.Where(listStu, (s) =>
                {
                    Console.WriteLine("123");
                    return s.age < 10;
                }).Select(s => new
                {
                    newId = s.id + "**",
                    newName = s.name + "###",
                    newAge = s.age + "++++"
                })
                .OrderBy(ns => ns.newId) //可做排序，正序

                .OrderByDescending(ns => ns.newAge) //倒序

                .ThenBy(ns=>ns.newAge) //再次排序

                .Skip(10)  //可做分页，跳过多少条数据

                .Take(3);  //获取多少条数据

                foreach (var item in list)
                {
                    Console.WriteLine($"该学生的年龄小于10的有{item.newId}==>{item.newName}===>{item.newAge}");
                }
            }

            {
                //内连接
                Console.WriteLine("**************************************");
                var list = Enumerable.Join(listStu, listClass, m => m.id, n => n.id, (m, n) => new
                {
                    stuId=m.id,
                    classId=n.id
                }).DefaultIfEmpty();   // 如果要写左连接，则直接在末尾加上DefaultIfEmpty(),去掉则为内连接

                foreach (var item in list)
                {
                    Console.WriteLine($"内连接：stuId={item.stuId},classId={item.classId}");
                }
            }


            Console.WriteLine("##############################################################################");

            {
                //查询表达式
                Console.WriteLine("**************************************");
                var list = from s in listStu
                           where s.age < 10
                           select s;
                foreach (var item in list)
                {
                    Console.WriteLine($"该学生的年龄小于10的有{item.name}==>{item.age}");
                }
            }

            {
                //投影第2种
                Console.WriteLine("**************************************");
                var list = from s in listStu
                           where s.age < 10
                           orderby s.age descending
                           select new   //挑选出学生年龄小于10的学生后，再创建 新的对象 进行投影，new{}为匿名对象
                           {
                               newId = s.id + "**",
                               newName=s.name+"###",
                               newAge=s.age+"++++"
                           };
                           
                //list为新对象集合
                foreach (var item in list)
                {
                    Console.WriteLine($"该学生的年龄小于10的有{item.newId}==>{item.newName}===>{item.newAge}");
                }
            }

            {
                //内连接第二种
                Console.WriteLine("**************************************");
                var list = from s in listStu
                           join c in listClass on s.id equals c.id
                           select new   //挑选出学生年龄小于10的学生后，再创建 新的对象 进行投影，new{}为匿名对象
                           {
                               newId = s.id +c.id,
                               newName = s.name + c.name,
                               newAge = s.age + c.number
                           };

                //list为新对象集合
                foreach (var item in list)
                {
                    Console.WriteLine($"该学生的年龄小于10的有{item.newId}==>{item.newName}===>{item.newAge}");
                }
            }

            {
                //左连接
                Console.WriteLine("**************************************");
                var list = from s in listStu
                           join c in listClass on s.id equals c.id
                           into scList //将连接放进一个全新的集合中
                           from sc in scList.DefaultIfEmpty()
                           select new  
                           {
                               Name = s.name,
                               ClassName=sc==null?"无班级":sc.name //c变sc,为空则用
                           };

                //list为新对象集合
                foreach (var item in list)
                {
                    Console.WriteLine($"该学生的年龄小于10的有{item.Name}==>{item.ClassName}");
                }
            }
        }
    }
}

下面是一些Model类

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

class ClassStu
    {
        public int id { get; set; }
        public string name { get; set; }
        public int number { get; set; }
    }