C#中IEnumerable.GroupBy()的简单使用

背景介绍

在实际项目中，对数据进行GroupBy肯定是常用需求之一，特别是采用EF等ORM框架后隔绝了用SQL语句直接操作数据，LINQ中的GroupBy肯定是要掌握的。

首先先对一个字段GroupBy，代码如下：

        static void Main(string[] args)
        {


            List<Phone> PhoneLists = new List<Phone>()
            {
                new Phone { Country = "中国", City = "北京", Name = "小米" },
                new Phone { Country = "中国",City = "北京",Name = "华为"},
                new Phone { Country = "中国",City = "北京",Name = "联想"},
                new Phone { Country = "中国",City = "台北",Name = "魅族"},
                new Phone { Country = "日本",City = "东京",Name = "索尼"},
                new Phone { Country = "日本",City = "大阪",Name = "夏普"},
                new Phone { Country = "日本",City = "东京",Name = "松下"},
                new Phone { Country = "美国",City = "加州",Name = "苹果"},
                new Phone { Country = "美国",City = "华盛顿",Name = "三星"},
                new Phone { Country = "美国",City = "华盛顿",Name = "HTC"}

            };
            var Lists = PhoneLists.GroupBy(p => p.Country);
            foreach (var list in Lists)
            {
                Console.WriteLine(list.Key + ":");
                foreach (var lis in list)
                {
                    Console.WriteLine($"{lis.Country} - {lis.City} - {lis.Name}");
                }
            }
            Console.Read();
        }

运行结果如下：

由于GroupBy支持的委托是Func<TSource,Tkey> ，返回值是IEnumerable<IGroup<Tkey，TSource>>类型，也就是我们可以自己指定任何想要的键，代码如下：

        static void Main(string[] args)
        {       
             List<Phone> PhoneLists = new List<Phone>()
            {
                new Phone { Country = "中国", City = "北京", Name = "小米" },
                new Phone { Country = "中国",City = "北京",Name = "华为"},
                new Phone { Country = "中国",City = "北京",Name = "联想"},
                new Phone { Country = "中国",City = "台北",Name = "魅族"},
                new Phone { Country = "日本",City = "东京",Name = "索尼"},
                new Phone { Country = "日本",City = "大阪",Name = "夏普"},
                new Phone { Country = "日本",City = "东京",Name = "松下"},
                new Phone { Country = "美国",City = "加州",Name = "苹果"},
                new Phone { Country = "美国",City = "华盛顿",Name = "三星"},
                new Phone { Country = "美国",City = "华盛顿",Name = "HTC"}

            };
            var Lists = PhoneLists.GroupBy(p => new GetGroupClass().GetGroup(p));
            foreach (var list in Lists)
            {
                Console.WriteLine(list.Key + ":");
                foreach (var lis in list)
                {
                    Console.WriteLine($"{lis.Country} - {lis.City} - {lis.Name}");
                }
            }
            Console.Read();
         }
        public class GetGroupClass
        {
            public string GetGroup(Phone phone)
            {
                string flag = string.Empty;
                if (phone.Country.Equals("中国") || phone.Country.Equals("日本"))
                {
                    flag = "亚洲";
                }
                else if (phone.Country.Equals("美国"))
                {
                    flag = "美洲";
                }
                return flag;
            }
        }

上述代码中，增加GetGroupClass 这个类，里面的GetGroup方法支持根据洲来归类，而非国家。运行效果如下图所示：

上述功能都是基于一个属性GroupBy()，多个属性GroupBy的代码如下图所示：

        static void Main(string[] args)
        {


            List<Phone> PhoneLists = new List<Phone>()
            {
                new Phone { Country = "中国", City = "北京", Name = "小米" },
                new Phone { Country = "中国",City = "北京",Name = "华为"},
                new Phone { Country = "中国",City = "北京",Name = "联想"},
                new Phone { Country = "中国",City = "台北",Name = "魅族"},
                new Phone { Country = "日本",City = "东京",Name = "索尼"},
                new Phone { Country = "日本",City = "大阪",Name = "夏普"},
                new Phone { Country = "日本",City = "东京",Name = "松下"},
                new Phone { Country = "美国",City = "加州",Name = "苹果"},
                new Phone { Country = "美国",City = "华盛顿",Name = "三星"},
                new Phone { Country = "美国",City = "华盛顿",Name = "HTC"}

            };

            var Lists = PhoneLists.GroupBy(p => new { p.Country, p.City });
            foreach (var list in Lists)
            {
                Console.WriteLine(list.Key + ":");
                foreach (var lis in list)
                {
                    Console.WriteLine($"{lis.Country} - {lis.City} - {lis.Name}");
                }
            }
            Console.Read();
       }

返回Lists的类型如下图所示：

由此可以GroupBy可以将匿名类型作为Key，这让我有所猜测GroupBy的原理是将返回类型中的属性通过反射在源类型找到并作为Key来实现分组的，我很希望看到微软的源代码。运行效果如下图所示：