背景介紹

在實(shí)際項(xiàng)目中，對(duì)數(shù)據(jù)進(jìn)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(shù)據(jù)，LINQ中的GroupBy肯定是要掌握的。

首先先對(duì)一個(gè)字段GroupBy，代碼如下：

static void Main(string[] args){List<Phone> PhoneLists = new List<Phone>(){new Phone { Country = "中國(guó)", City = "北京", Name = "小米" },new Phone { Country = "中國(guó)",City = "北京",Name = "華為"},new Phone { Country = "中國(guó)",City = "北京",Name = "聯(lián)想"},new Phone { Country = "中國(guó)",City = "臺(tái)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(guó)",City = "加州",Name = "蘋果"},new Phone { Country = "美國(guó)",City = "華盛頓",Name = "三星"},new Phone { Country = "美國(guó)",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(yùn)行結(jié)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(guó)", City = "北京", Name = "小米" },new Phone { Country = "中國(guó)",City = "北京",Name = "華為"},new Phone { Country = "中國(guó)",City = "北京",Name = "聯(lián)想"},new Phone { Country = "中國(guó)",City = "臺(tái)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(guó)",City = "加州",Name = "蘋果"},new Phone { Country = "美國(guó)",City = "華盛頓",Name = "三星"},new Phone { Country = "美國(guó)",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(guó)") || phone.Country.Equals("日本")){flag = "亞洲";}else if (phone.Country.Equals("美國(guó)")){flag = "美洲";}return flag;}}

上述代碼中，增加GetGroupClass 這個(gè)類，里面的GetGroup方法支持根據(jù)洲來歸類，而非國(guó)家。運(yùn)行效果如下圖所示：

上述功能都是基于一個(gè)屬性GroupBy()，多個(gè)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(guó)", City = "北京", Name = "小米" },new Phone { Country = "中國(guó)",City = "北京",Name = "華為"},new Phone { Country = "中國(guó)",City = "北京",Name = "聯(lián)想"},new Phone { Country = "中國(guó)",City = "臺(tái)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(guó)",City = "加州",Name = "蘋果"},new Phone { Country = "美國(guó)",City = "華盛頓",Name = "三星"},new Phone { Country = "美國(guó)",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(cè)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(shí)現(xiàn)分組的，我很希望看到微軟的源代碼。運(yùn)行效果如下圖所示：

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