C# Linq Jion用法

没用过数据库,对这个扩展方法不是很理解，今天记录一下 大概意思就是在A,B两个序列中，根据条件查找相同的项

Msdn官方介绍 Join 操作（C#） | Microsoft Docs

1. Join 和 GroupJoin是连接运算符。

2. Join 类似于SQL的内部连接。它返回一个新集合，其中包含两个键匹配的集合中的公共元素。

3. Join 对内部序列和外部序列这两个序列进行运算，并生成结果序列。

数据库中有左连接和右连接，linq就一个Jion（反过来操作即可实现右连接) 先看简单例子

 

 

再看复杂一点的

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

namespace ConsoleApp5
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var list1 = new List<Student>();
            list1.Add(new Student() { Id = 1, Name = "韩梅梅" });
            list1.Add(new Student() { Id = 2, Name = "李磊" });
            list1.Add(new Student() { Id = 3, Name = "张三", Age = 15 });
            list1.Add(new Student() { Id = 4, Name = "刘能" });
            list1.Add(new Student() { Id = 4, Name = "李四" });
            var list2 = new List<Student>();
            list2.Add(new Student() { Id = 2, Name = "韩梅梅" });
            list2.Add(new Student() { Id = 1, Name = "李磊" });
            list2.Add(new Student() { Id = 3, Name = "张三", Age = 18 });
            list2.Add(new Student() { Id = 7, Name = "王五" });
            list2.Add(new Student() { Id = 8, Name = "李四" });
            list2.Add(new Student() { Id = 4, Name = "赵六" });
            //查询list1中与list2中 Name 相同的项
            var query1 = list1.Join(list2, s => s.Name, s => s.Name, (s1, s2) => s1);
            foreach (var stu in query1)
            {
                Console.WriteLine($"{stu.Id}---{stu.Name}");
            }

            Console.WriteLine("===================");
            //查询list2中Id与list1相同的项，
            //可以注意到，结果的顺序是按照list1中查询到的顺序排列
            var query2 = list1.Join(list2, s => s.Id, s => s.Id, (s1, s2) => s2);
            foreach (var stu in query2)
            {
                Console.WriteLine($"{stu.Id}---{stu.Name}");
            }

            Console.WriteLine("===================");
            var query3 = list1.Join(list2, s => new { s.Id, s.Name }, s => new { s.Id, s.Name }, (s1, s2) => s1);
            //查询list1中Id,Name在list2中都相同的项
            foreach (var stu in query3)
            {
                Console.WriteLine($"{stu.Id}---{stu.Name}---{stu.Age}");
            }
        }
    }

    internal class Student
    {
        public int Age { get; set; }
        public int Id { get; set; }
        public string Name { get; set; }
    }
}

不同类型的查询

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

namespace ConsoleApp5
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var list1 = new List<Student>();
            list1.Add(new Student() { Id = 1, Name = "韩梅梅" });
            list1.Add(new Student() { Id = 2, Name = "李磊" });
            list1.Add(new Student() { Id = 3, Name = "张三", Age = 15 });
            list1.Add(new Student() { Id = 4, Name = "刘能" });
            list1.Add(new Student() { Id = 4, Name = "李四" });
            var list2 = new List<Standard>();
            list2.Add(new Standard() { Id = 1, Name = "一年级" });
            list2.Add(new Standard() { Id = 2, Name = "二年级" });
            list2.Add(new Standard() { Id = 3, Name = "三练级" });
            //查询list1中与list2中id相同的项,并输出到一个匿名类
            var query1 = list1.Join(
                list2,
                stu => stu.Id,
                sta => sta.Id,
                (s1, s2) => new { s1.Id, s1.Name, Standard = s2.Name });
            foreach (var item in query1)
            {
                Console.WriteLine($"{item.Id}---{item.Name}----{item.Standard}");
            }
        }
    }

    internal class Standard
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    internal class Student
    {
        public int Age { get; set; }
        public int Id { get; set; }
        public string Name { get; set; }
    }
}

 

 GroupJion

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

namespace ConsoleApp5
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var list1 = new List<Student>();
            list1.Add(new Student() { Id = 1, Name = "韩梅梅" });
            list1.Add(new Student() { Id = 2, Name = "李磊" });
            list1.Add(new Student() { Id = 2, Name = "李雷" });
            list1.Add(new Student() { Id = 2, Name = "李蕾" });
            list1.Add(new Student() { Id = 3, Name = "张三", Age = 15 });
            list1.Add(new Student() { Id = 3, Name = "张散", Age = 15 });
            list1.Add(new Student() { Id = 3, Name = "张叁", Age = 15 });
            list1.Add(new Student() { Id = 4, Name = "刘能" });
            list1.Add(new Student() { Id = 4, Name = "李四" });
            var list2 = new List<Standard>();
            list2.Add(new Standard() { Id = 2, Name = "二年级" });
            list2.Add(new Standard() { Id = 3, Name = "三练级" });
            list2.Add(new Standard() { Id = 5, Name = "三练级" });
            //按年级分类学生列表
            var query1 = list2.GroupJoin(
                list1,
                stu => stu.Id,
                sta => sta.Id,
                (s1, s2) => new { Standard = s1, Students = s2 });
            foreach (var item in query1)
            {
                Console.WriteLine($"{item.Standard.Name}");
                foreach (var stu in item.Students)
                {
                    Console.WriteLine($"{stu.Name}");
                }
                Console.WriteLine("");
            }
        }
    }

    internal class Standard
    {
        public int Id { get; set; }
        public string Name { get; set; }
    }

    internal class Student
    {
        public int Age { get; set; }
        public int Id { get; set; }
        public string Name { get; set; }
    }
}

 

 

 另外还有一个方法重载，传入IEqualityComparer比较器 用法和这个类似