Enumerable<T>的GroupJoin 方法和Join的用法和区别

平时在项目中很多时候直接用 from s in list1 join s2 in list2 ....这样的写法，最近在项目中看到有同事用了Enumerable<T>的GroupJoin 方法和Join的用法，我也没有深究二者的区别，今天就做了一个总结。

首先我们 先看join 的用法：

 class Person
    {
        public string Name { get; set; }
    }

    class Pet
    {
        public string Name { get; set; }
        public Person Owner { get; set; }
    }
    class Program
    {
        private static readonly int[] nums = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
        static void Main(string[] args)
        {
            Person magnus = new Person { Name = "Hedlund, Magnus" };
            Person terry = new Person { Name = "Adams, Terry" };
            Person charlotte = new Person { Name = "Weiss, Charlotte" };

            Pet barley = new Pet { Name = "Barley", Owner = terry };
            Pet boots = new Pet { Name = "Boots", Owner = terry };
            Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
            Pet daisy = new Pet { Name = "Daisy", Owner = new Person() };

            List<Person> people = new List<Person> { magnus, terry, charlotte };
            List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
            var query = people.Join
                   (pets,
                    person => person,
                    pet => pet.Owner,
                    (person, pet) =>
                        new
                        {
                            OwnerName = person.Name,
                            Pet = pet.Name
                        });

            foreach (var obj in query)
            {
                Console.WriteLine(
                    "{0} - {1}",
                    obj.OwnerName,
                    obj.Pet);
            }
            Console.ReadKey();
        }

我们断点调试的时候可以看到:

控制台输出的结果：

从上面的结果我们可以看出Join是一个inner join的结果，同时这个结果集是一个一维的平面结果集。

还是相同的数据，我们现在把代码改成GroupJoin的写法如下：

            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();

我们调试的时候可以看到如下：

再看看输出的结果

我们可以看出GroupJoin是一个left join的结果，是一个分组后的二维结果集