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
)

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。
返回值
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。

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。
TKey
键选择器函数返回的键的类型。
TResult
结果元素的类型。

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}";
 }
 }
}

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
)

outer
Type: System.Collections.Generic.IEnumerable<TOuter>

inner
Type: System.Collections.Generic.IEnumerable<TInner>

outerKeySelector
Type: System.Func<TOuter, TKey>

innerKeySelector
Type: System.Func<TInner, TKey>

resultSelector
Type: System.Func<TOuter, IEnumerable<TInner>, TResult>

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。

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。
TKey
键选择器函数返回的键的类型。
TResult
结果元素的类型。

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} ");
   else
   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; }
 public override string ToString()
 {
  return $"{Name}";
 }
 }
}