背景介紹

在實際項目中，對數據進行GroupBy肯定是常用需求之一，特別是采用EF等ORM框架后隔絕了用SQL語句直接操作數據，LINQ中的GroupBy肯定是要掌握的。

首先先對一個字段GroupBy，代碼如下：

static void Main(string[] args){List<Phone> PhoneLists = new List<Phone>(){new Phone { Country = "中國", City = "北京", Name = "小米" },new Phone { Country = "中國",City = "北京",Name = "華為"},new Phone { Country = "中國",City = "北京",Name = "聯想"},new Phone { Country = "中國",City = "臺北",Name = "魅族"},new Phone { Country = "日本",City = "東京",Name = "索尼"},new Phone { Country = "日本",City = "大阪",Name = "夏普"},new Phone { Country = "日本",City = "東京",Name = "松下"},new Phone { Country = "美國",City = "加州",Name = "蘋果"},new Phone { Country = "美國",City = "華盛頓",Name = "三星"},new Phone { Country = "美國",City = "華盛頓",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();}

運行結果如下：

由于GroupBy支持的委托是Func<TSource,Tkey> ，返回值是IEnumerable<IGroup<Tkey，TSource>>類型，也就是我們可以自己指定任何想要的鍵，代碼如下：

static void Main(string[] args){ List<Phone> PhoneLists = new List<Phone>(){new Phone { Country = "中國", City = "北京", Name = "小米" },new Phone { Country = "中國",City = "北京",Name = "華為"},new Phone { Country = "中國",City = "北京",Name = "聯想"},new Phone { Country = "中國",City = "臺北",Name = "魅族"},new Phone { Country = "日本",City = "東京",Name = "索尼"},new Phone { Country = "日本",City = "大阪",Name = "夏普"},new Phone { Country = "日本",City = "東京",Name = "松下"},new Phone { Country = "美國",City = "加州",Name = "蘋果"},new Phone { Country = "美國",City = "華盛頓",Name = "三星"},new Phone { Country = "美國",City = "華盛頓",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("中國") || phone.Country.Equals("日本")){flag = "亞洲";}else if (phone.Country.Equals("美國")){flag = "美洲";}return flag;}}

上述代碼中，增加GetGroupClass 這個類，里面的GetGroup方法支持根據洲來歸類，而非國家。運行效果如下圖所示：

上述功能都是基于一個屬性GroupBy()，多個屬性GroupBy的代碼如下圖所示：

static void Main(string[] args){List<Phone> PhoneLists = new List<Phone>(){new Phone { Country = "中國", City = "北京", Name = "小米" },new Phone { Country = "中國",City = "北京",Name = "華為"},new Phone { Country = "中國",City = "北京",Name = "聯想"},new Phone { Country = "中國",City = "臺北",Name = "魅族"},new Phone { Country = "日本",City = "東京",Name = "索尼"},new Phone { Country = "日本",City = "大阪",Name = "夏普"},new Phone { Country = "日本",City = "東京",Name = "松下"},new Phone { Country = "美國",City = "加州",Name = "蘋果"},new Phone { Country = "美國",City = "華盛頓",Name = "三星"},new Phone { Country = "美國",City = "華盛頓",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的類型如下圖所示：

由此可以GroupBy可以將匿名類型作為Key，這讓我有所猜測GroupBy的原理是將返回類型中的屬性通過反射在源類型找到并作為Key來實現分組的，我很希望看到微軟的源代碼。運行效果如下圖所示：

總結

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