Enumerable<T>的GroupJoin 方法和Jo的用法和區別

阿新 • • 發佈：2020-11-05

平時在專案中很多時候直接用 from s in list1 join s2 in list2 ....這樣的寫法，最近在專案中看到有同事用了Enumerable<T>的GroupJoin 方法和Join的用法，我也沒有深究二者的區別，今天就做了一個總結。

首先我們先看join 的用法：

 1  class Person
 2     {
 3         public string Name { get; set; }
 4     }
 5 
 6     class Pet
 7     {
 8         public string Name { get; set 
; }
 9         public Person Owner { get; set; }
10     }
11     class Program
12     {
13         private static readonly int[] nums = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
14         static void Main(string[] args)
15         {
28             Person magnus = new Person { Name = "Hedlund, Magnus 
" };
29             Person terry = new Person { Name = "Adams, Terry" };
30             Person charlotte = new Person { Name = "Weiss, Charlotte" };
31 
32             Pet barley = new Pet { Name = "Barley", Owner = terry };
33             Pet boots = new Pet { Name = "Boots", Owner = terry };
34             Pet whiskers = new 
 Pet { Name = "Whiskers", Owner = charlotte };
35             Pet daisy = new Pet { Name = "Daisy", Owner = new Person() };
36 
37             List<Person> people = new List<Person> { magnus, terry, charlotte };
38             List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
65             var query = people.Join
66                    (pets,
67                     person => person,
68                     pet => pet.Owner,
69                     (person, pet) =>
70                         new
71                         {
72                             OwnerName = person.Name,
73                             Pet = pet.Name
74                         });
75 
76             foreach (var obj in query)
77             {
78                 Console.WriteLine(
79                     "{0} - {1}",
80                     obj.OwnerName,
81                     obj.Pet);
82             }
83             Console.ReadKey();
84         }

我們斷點除錯的時候可以看到:

控制檯輸出的結果：

從上面的結果我們可以看出Join是一個inner join的結果，同時這個結果集是一個一維的平面結果集。

還是相同的資料，我們現在把程式碼改成GroupJoin的寫法如下：

            var query =
                people.GroupJoin(pets,
                                 person => person,
                                 pet => pet.Owner,
                                 (person, petCollection) =>
                                     new
                                     {
                                         OwnerName = person.Name,
                                         Pets = petCollection.Select(pet => pet.Name),
                                         Count = petCollection.Count()
                                     });

            foreach (var obj in query)
            {
                // Output the owner's name.
                Console.WriteLine("{0}:", obj.OwnerName);
                // Output each of the owner's pet's names.
                foreach (string pet in obj.Pets)
                {
                    Console.WriteLine("  {0}", pet);
                }
            }
            Console.ReadKey();