LINQ~什么时候使用SelectMany和GroupBy

  class PetOwner
    {
        public string Name { get; set; }
        public List<Pet> Pet { get; set; }
    }

    class Pet
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public int Age { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            PetOwner[] petOwners =
                    { new PetOwner { Name="zzl",
                          Pet = new List<Pet>{new Pet{Id=1,Name="小狗"},new Pet{Id=2,Name="小猫"},new Pet{Id=3,Name="小老虎"} } },
                      new PetOwner { Name="zql",
                          Pet = new List<Pet>{new Pet{Id=1,Name="小狗"} } },
                      new PetOwner { Name="zhz",
                          Pet = new List<Pet>{new Pet{Id=1,Name="小狗"},new Pet{Id=3,Name="小老虎"} } },
                    };

            List<Pet> PetList = new List<Pet>
            {
                new Pet { Id = 1, Name = "小狗",Age=1 },
                new Pet { Id = 2, Name = "小猫",Age=2 },
                new Pet { Id = 3, Name = "小老虎",Age=1},
                new Pet { Id = 4, Name = "小蛇",Age=3 },
                new Pet { Id = 5, Name = "小蝎子",Age=2 },
                new Pet { Id = 6, Name = "小兔子",Age=3},
            };

            #region 不知道SelectMany之前，选择LIST是多么的悲哀
            List<Pet> query2 = new List<Pet>();
            petOwners.AsQueryable().Where(i => i.Name == "zzl").ToList().ForEach(i =>
            {
                query2.AddRange(i.Pet);
            });
            foreach (Pet pet in query2)
                Console.WriteLine(pet.Name);
            #endregion

            #region 有了SelectMany，我们的代码的可读及执行效率上都有所提高
            List<Pet> query1 = petOwners.AsQueryable().Where(i => i.Name == "zzl")                               .SelectMany(i => i.Pet).ToList();
            query1.ForEach(i => Console.WriteLine(i.Name));
            #endregion

            #region GroupBy 在集合中根据某个字段进行分组
            var query = PetList.AsQueryable().GroupBy(pet => pet.Age);
            foreach (var ageGroup in query)
            {
                Console.WriteLine("Pet s' Age group: {0}  Number of pets: {1}",                ageGroup.Key, ageGroup.Count());
            }
            #endregion

            #region 对age属性进行分组，并把name显示出来
            IEnumerable<IGrouping<int, Pet>> query3 =
             PetList.AsQueryable().GroupBy(pet => pet.Age, pet => pet);

            foreach (IGrouping<int, Pet> petGroup in query3)
            {
                Console.WriteLine(petGroup.Key);
                foreach (Pet pet in petGroup)
                    Console.WriteLine("  {0}", pet.Id+"..."+pet.Name);
            }
            #endregion


            Console.ReadKey();
        }
    }