C# LINQ

Posted 2021-04-14 HelloCopyer

LINQ允许我们以使用SQL查询数据库的方式来查询数据集合。下面我们查看一段代码，代码演示了如何使用LINQ的示例。

static void Main(string[] args) {     int[] numbers = { 1, 2, 3, 4,5,6,7,8 };         //数据源     IEnumerable<int> lowNums =                //定义并存储查询                  from n in numbers                  where n < 4                  select n;                   foreach (var x in lowNums) //执行查询            Console.Write("{0}, ", x);  }

代码产生了如下输出：

1, 2, 3,

语句介绍

• from

• join

• let

• where

• orderby

• group

from

from子句是最基本的，基本语法如下：from 变量 in 集合

 int[] arr = { 1, 2, 3 }; var query = from n in arr where n < 2  select n;  foreach (var item in query) Console.WriteLine(item);

输出结果

1

join

• 连接多个集合的数据

• 连接

 public class A {    public string name;    public string id; }
 public class B {    public string sex;    public string id; }
 static A[] aS = new A[] {    new A{ name = "小明",id = "01"},    new A{ name = "小白",id = "02"},    new A{ name = "小黑",id = "03"},  };
 static B[] bS = new B[] {    new B{ sex = "男",id = "01"},    new B{ sex = "男",id = "02"},    new B{ sex = "女",id = "03"}, };
 static void Main(string[] args) {    var quert = from a in aS          join b in bS on a.id equals b.id          where b.sex == "男"          select a.name;                     foreach (var item in quert)           {               Console.WriteLine("性别为男的姓名：" + item);           }           Console.ReadKey(); }

输出结果：

性别为男的姓名：小明
性别为男的姓名：小白

let

static void Main(){ var groupA = new[] { 3, 4, 5, 6 }; var groupB = new[] { 6, 7, 8, 9 };
 var someInts = from a in groupA from b in groupB let sum = a + b ←在新的变量中保存结果 where sum == 12 select new {a, b, sum};
 foreach (var a in someInts) Console.WriteLine(a);}

输出结果

{ a = 3, b = 9, sum = 12 }
{ a = 4, b = 8, sum = 12 }
{ a = 5, b = 7, sum = 12 }
{ a = 6, b = 6, sum = 12 }

where

static void Main(){ var groupA = new[] { 3, 4, 5, 6 }; var groupB = new[] { 6, 7, 8, 9 };
 var someInts = from int a in groupA from int b in groupB let sum = a + b where sum >= 11 ←条件1 where a == 4 ←条件2 select new {a, b, sum};
 foreach (var a in someInts) Console.WriteLine(a);}

输出结果

{ a = 4, b = 7, sum = 11 }
{ a = 4, b = 8, sum = 12 }
{ a = 4, b = 9, sum = 13 }

orderby

static void Main( ) { var students = new [] //匿名类型的对象数组 { new { LName="Jones", FName="Mary", Age=19, Major="History" }, new { LName="Smith", FName="Bob", Age=20, Major="CompSci" }, new { LName="Fleming", FName="Carol", Age=21, Major="History" } };
 var query = from student in students orderby student.Age ←根据Age排序 select student;
 foreach (var s in query) { Console.WriteLine("{0}, {1}: {2} - {3}", s.LName, s.FName, s.Age, s.Major); }}

输出结果：

Jones, Mary:  19 – HistorySmith,
Bob:  20 – CompSciFleming,
Carol:  21 – History

group

static void Main( ){ var students = new[] //匿名类型的对象数组 { new { LName="Jones", FName="Mary", Age=19, Major="History" }, new { LName="Smith", FName="Bob", Age=20, Major="CompSci" }, new { LName="Fleming", FName="Carol", Age=21, Major="History" } };
 var query = from student in students group student by student.Major;
 foreach (var s in query) //枚举分组 { Console.WriteLine("{0}", s.Key); --↑-- 分组键 foreach (var t in s) //枚举分组中的项 Console.WriteLine(" {0}, {1}", t.LName, t.FName); }}

输出结果：

History
      Jones, Mary
      Fleming,
CarolCompSci
      Smith, Bob