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本文使用的开发环境是VS2017及dotNet4.0，写此随笔的目的是给自己及新开发人员作为参考，

对于Join的用法说明如下：

语法：

public static IEnumerable<TResult> Join<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer,
    IEnumerable<TInner> inner,
    Func<TOuter, TKey> outerKeySelector,
    Func<TInner, TKey> innerKeySelector,
    Func<TOuter, TInner, TResult> resultSelector
)

参数说明：

outer
Type: System.Collections.Generic.IEnumerable<TOuter>
要联接的第一个序列。
inner
Type: System.Collections.Generic.IEnumerable<TInner>
要与第一个序列联接的序列。
outerKeySelector
Type: System.Func<TOuter, TKey>
用于从第一个序列的每个元素提取联接键的函数。
innerKeySelector
Type: System.Func<TInner, TKey>
用于从第二个序列的每个元素提取联接键的函数。
resultSelector
Type: System.Func<TOuter, TInner, TResult>
用于从两个匹配元素创建结果元素的函数。
Type: System.Collections.Generic.IEnumerable<TResult>
IEnumerable<T> ，其类型的元素 TResult 通过对两个序列执行内部联接获得的。

参数类型：

TOuter
第一个序列中的元素的类型。
TInner
第二个序列中的元素的类型。
键选择器函数返回的键的类型。
TResult
结果元素的类型。 

参考链接如下：

https://msdn.microsoft.com/zh-cn/library/bb534675.aspx

https://docs.microsoft.com/zh-cn/dotnet/api/system.linq.enumerable.join?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DZH-CN%26k%3Dk(System.Linq.Enumerable.Join%60%604);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.0);k(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.1 

例程：

using System;
using System.Collections.Generic;
using System.Linq;
namespace ConsoleApp33
    class Program
        static void Main(string[] args)
            GroupJoinEx();
        static void GroupJoinEx()
            Person p1 = new Person() { Name = "ABC", Age = 18 };
            Person p2 = new Person() { Name = "EFG", Age = 19 };
            Person p3 = new Person() { Name = "LMN", Age = 20 };
            Person p4 = new Person() { Name = "XYZ", Age = 21 };
            List<Person> pList = new List<Person> { p1, p2, p3, p4 };
            Department d1 = new Department() { Name = "A1", Employee = p1 };
            Department d2 = new Department() { Name = "A2", Employee = p2 };
            Department d3 = new Department() { Name = "A3", Employee = p1 };
            Department d4 = new Department() { Name = "B1", Employee = p3 };
            Department d5 = new Department() { Name = "B2", Employee = p4 };
            Department d6 = new Department() { Name = "B3", Employee = p4 };
            List<Department> dList = new List<Department> { d1, d2, d3, d4, d5, d6 };
            var result = pList.Join(dList,
                person => person,
                department => department.Employee,
                (person, department) => new
                    Person = person,
                    Department = department
            foreach(var item1 in result)
                Console.Write($"Name:{item1.Person} & Department:{item1.Department} ");
                Console.WriteLine();
    class Person
        public string Name { set; get; }
        public int Age { set; get; }
        public override string ToString()
            return $"{Name},{Age}";
    class Department
        public string Name { set; get; }
        public Person Employee { set; get; }
        public override string ToString()
            return $"{Name}";
}

输出结果：

对于GroupJoin的用法说明如下：

语法：

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
)

参数说明：

outer
Type: System.Collections.Generic.IEnumerable<TOuter>
要联接的第一个序列。
inner
Type: System.Collections.Generic.IEnumerable<TInner>
要与第一个序列联接的序列。
outerKeySelector
Type: System.Func<TOuter, TKey>
用于从第一个序列的每个元素提取联接键的函数。
innerKeySelector
Type: System.Func<TInner, TKey>
用于从第二个序列的每个元素提取联接键的函数。
resultSelector
Type: System.Func<TOuter, IEnumerable<TInner>, TResult>
用于从第一个序列的元素和第二个序列的匹配元素集合中创建结果元素的函数。
Type: System.Collections.Generic.IEnumerable<TResult>
IEnumerable<T> ，其中包含类型的元素 TResult 通过对两个序列执行分组的联接获得的。

参数类型：

TOuter
第一个序列中的元素的类型。
TInner
第二个序列中的元素的类型。
键选择器函数返回的键的类型。
TResult
结果元素的类型。

参考链接如下：

https://msdn.microsoft.com/zh-cn/library/bb534297.aspx

https://docs.microsoft.com/zh-cn/dotnet/api/system.linq.enumerable.groupjoin?f1url=https%3A%2F%2Fmsdn.microsoft.com%2Fquery%2Fdev15.query%3FappId%3DDev15IDEF1%26l%3DZH-CN%26k%3Dk(System.Linq.Enumerable.GroupJoin%60%604);k(TargetFrameworkMoniker-.NETFramework,Version%3Dv4.0);k(DevLang-csharp)%26rd%3Dtrue&view=netframework-4.7.1

例程：

using System;
using System.Collections.Generic;
using System.Linq;
namespace ConsoleApp33
    class Program
        static void Main(string[] args)
            GroupJoinEx();
        static void GroupJoinEx()
            Person p1 = new Person() { Name = "ABC", Age = 18 };
            Person p2 = new Person() { Name = "EFG", Age = 19 };
            Person p3 = new Person() { Name = "LMN", Age = 20 };
            Person p4 = new Person() { Name = "XYZ", Age = 21 };
            List<Person> pList = new List<Person> { p1, p2, p3, p4 };
            Department d1 = new Department() { Name = "A1", Employee = p1 };
            Department d2 = new Department() { Name = "A2", Employee = p2 };
            Department d3 = new Department() { Name = "A3", Employee = p1 };
            Department d4 = new Department() { Name = "B1", Employee = p3 };
            Department d5 = new Department() { Name = "B2", Employee = p4 };
            Department d6 = new Department() { Name = "B3", Employee = p4 };
            List<Department> dList = new List<Department> { d1, d2, d3, d4, d5, d6 };
            var result = pList.GroupJoin(dList,
                person => person,
                department => department.Employee,
                (person, departments) => new
                    Person = person,
                    Department = departments.Select(d => d)
            foreach(var item1 in result)
                Console.Write($"Name:{item1.Person} & ");
                foreach(var item2 in item1.Department)
                    if(item1.Department.First() == item2)
                        Console.Write($"Department:{item2} ");
                        Console.Write($"{item2} ");
                Console.WriteLine();
    class Person
        public string Name { set; get; }
        public int Age { set; get; }
        public override string ToString()
            return $"{Name},{Age}";
    class Department
        public string Name { set; get; }
        public Person Employee { set; get; }