Linq inner join & left join & group join

今天写了一段linq，感觉还是很生疏，贴出来记录一下！

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

namespace ConsoleApp
{
    class Employee
    {
        public int ID { set; get;}
        public string Name { set; get; }
        public int Age { set; get; }

        public int RoleId { set; get; }
    }

    class Role
    {
        public int RoleId { set; get; }
        public string RoleName { set; get; }
    }

    class Program
    {

        static void Main(string[] args)
        {
            List<Employee> listEmployee = new List<Employee>() 
            {
                new Employee{ ID = 1, Name = "Employee1", Age = 20,RoleId = 1},
                new Employee{ ID = 2, Name = "Employee2", Age = 21,RoleId = 2},
                new Employee{ ID = 3, Name = "Employee3", Age = 22,RoleId = 2},
                new Employee{ ID = 4, Name = "Employee4", Age = 23,RoleId = 2},
                new Employee{ ID = 5, Name = "Employee5", Age = 24,RoleId = 2}
            };

            List<Role> listRole = new List<Role>() 
            {
                new Role{ RoleId = 1, RoleName = "Admin"},
                new Role{ RoleId = 2, RoleName = "Guest"},
                new Role{ RoleId = 3, RoleName = "None"}
            };

            var q = from e in listEmployee
                    where e.Age == 20
                    select new Employee { ID = 6, Age = e.Age, Name = "EmployeeNew" };

            foreach (var item in q)
            {
                Console.WriteLine("id:" + item.ID + ",name:" + item.Name + ",age:" + item.Age + ".");
            }

            //inner join
            var p = from r in listRole
                    join e in listEmployee on r.RoleId equals e.RoleId where r.RoleId == 1
                    select new { e.Name, e.RoleId };

            foreach (var item in p)
            {
                Console.WriteLine("RoleId:" + item.RoleId + ",name:" + item.Name + ".");
            }

            //group join
            var query = from r in listRole 
                        join e in listEmployee on r.RoleId equals e.RoleId into ss where r.RoleId == 1
                        select new { r.RoleId,ss };

            foreach (var item in query)
            {
                Console.Write("RoleId:" + item.RoleId + ",");
                foreach (var s in item.ss)
                {
                    Console.WriteLine("name:" + s.Name);
                }
            }

            //left join
            var query2 = from r in listRole
                         join e in listEmployee on r.RoleId equals e.RoleId into ss where r.RoleId == 1
                         from s in ss.DefaultIfEmpty(new Employee { ID = 0, Name = string.Empty, Age = 0, RoleId = 0 })
                         select new { r, ss };

            foreach (var item in query2)
            {
                Console.Write("RoleId:" + item.r.RoleId + ",");
                foreach (var s in item.ss)
                {
                    Console.WriteLine("name:" + s.Name);
                }
            }

            Console.ReadKey();
        }
    }
}