C# LINQ中Join与GroupJoin的区别

C# LINQ中Join与GroupJoin的区别

新建控制台项目ConsoleDemo，添加Testlinq类，代码如下：

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

namespace ConsoleDemo
{
    public class Testlinq
    {
        private List<UserDepart> userDeparts = new List<UserDepart> { };
        private List<NoticeData> noticeDatas = new List<NoticeData> { };
        public Testlinq()
        {
            for (int i = 0; i < 10; i++)
            {
                userDeparts.Add(new UserDepart { UserId = i + 1, Name = $"机构：{i + 1}", DepartmentId = i + 1 });
                if (i % 2 == 0)
                {
                    noticeDatas.Add(new NoticeData { Id = i + 1, Name = $"公告：{i + 1}", DepartmentId = i + 1 });
                }
            }
        }
        public void Testjoin()
        {
            //查询语法
            var x = from a in userDeparts
                    join b in noticeDatas on a.DepartmentId equals b.DepartmentId
                    select new { name = a.Name, id = a.DepartmentId };
            Console.WriteLine("输出连接信息如下：");
            foreach (var item in x)
            {
                Console.WriteLine($"{item.name},{item.id}");
            }

            //函数语法
            //var s = userDeparts.Join(noticeDatas, a => a.DepartmentId, b => b.DepartmentId, (a, b) => new { name = a.Name, id = a.DepartmentId });
            //Console.WriteLine("输出连接信息如下：");
            //foreach (var item in s)
            //{
            //    Console.WriteLine($"{item.name},{item.id}");
            //}            
        }

        public void TestGroupjoin()
        {
            //查询语法
            var y = from a in userDeparts
                    join b in noticeDatas on a.DepartmentId equals b.DepartmentId
                    into clist
                    select new { name = a.Name, list = clist };
            Console.WriteLine("输出连接信息如下：");
            foreach (var item in y)
            {
                Console.Write($"{item.name}，内容如下：");
                foreach (var subitem in item.list)
                {
                    Console.Write($"{subitem.Name},{subitem.DepartmentId}");
                }
                Console.WriteLine();
            }
            //函数语法
            var s = this.userDeparts.GroupJoin(noticeDatas, a => a.DepartmentId, b => b.DepartmentId, (a, blist) => new { name = a.Name, list = blist });
            //Console.WriteLine("输出连接信息如下：");
            //foreach (var item in s)
            //{
            //    Console.Write($"{item.name}，内容如下：");
            //    foreach (var subitem in item.list)
            //    {
            //        Console.Write($"{subitem.Name},{subitem.DepartmentId}");
            //    }
            //    Console.WriteLine();
            //}

            //从结果来看 
            //1、GroupJoin 和数据库的 left outer join 一样
            //public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>(this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
            //2、Func<TOuter, IEnumerable<TInner>, TResult> resultSelector 也就是 GroupJoin中的(a, blist)  blist类型是 IEnumerable<TInner>
        }
    }
    public class UserDepart
    {
        public long UserId { get; set; }
        public long DepartmentId { get; set; }
        public string Name { get; set; }

    }
    public class NoticeData
    {
        public long Id { get; set; }
        public long DepartmentId { get; set; }
        public string Name { get; set; }
    }
}

控制台项目ConsoleDemo的Program添加 如下：

   Testlinq testlinq = new Testlinq();
            testlinq.Testjoin();
            testlinq.TestGroupjoin();

　　

运行结果：