LINQ 101——约束、投影、排序

什么是LINQ：LINQ 是一组 .NET Framework 扩展模块集合，内含语言集成查询、集合以及转换操作。它使用查询的本机语言语法来扩展 C# 和 Visual Basic，并提供利用这些功能的类库。

什么是LINQ 101：是学习LINQ的不错的资源（下载地址：http://pan.baidu.com/s/1ntE74NJ），当初我也是看LINQ101来熟悉LINQ的。现把我当时的学习笔记整理出来

Restriction （约束)
Projection （投影）
Ordering （排序）
Partitioning （分区）
Join
Aggregate(聚合)
Grouping（分组）
Set(集合)
Conversion（转换）
Element（元素操作）

本篇文章介绍前三个

一、 约束 ——where

var query = from item in [集合] where [item满足的条件] select item;

int[] numbers = { 5, 4, 3, 7, 9, -12 };

var query = from n in numbers

            where n % 3 == 0

            select n;

Console.WriteLine("被3整除的数：");

foreach (var num in numbers)

{

    Console.WriteLine(num);

}

View Code

 

多个Where 条件且筛选

 1 var query = from item in sources

 2             where (字段1的条件1) || (字段1的条件2)

 3             where (字段2的条件1) || (字段2的条件2)

 4             select new Entity ....

 5 

 6 如 

 7 var query = from item in sources

 8             where (string.IsNullOrEmpty(txtSearch.Text.Trim())) || (item.Filed1.Contains(txtSearch.Text.Trim()))

 9             where (string.IsNullOrEmpty(txtFilter.Text.Trim())) || (item.Filed2.Contains(txtFilter.Text.Trim()))

10             ...

11             select new Entity ....

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二、投影——select

例1：让数组中每个数都加1

 1 int[] numbers = { 5, 4, 1, 3, 9, 8,-13 };

 2 

 3 var numsPlusOne =

 4     from n in numbers

 5     select n + 1;

 6 

 7 Console.WriteLine("整数+1:");

 8 foreach (var num in numsPlusOne)

 9 {

10     Console.WriteLine(num);

11 }

View Code

例2：选取某类中一个属性

 1 public class Config

 2 {

 3     public Guid ID { get; set; }

 4     public string Name { get; set; }

 5     public string Remark { get; set; }

 6 

 7     Config() { }

 8 

 9     public static List<Config> GetConfigs()

10     {

11         return new List<Config> 

12         { 

13             new Config{ ID=Guid.NewGuid(), Name="XX1号",Remark="Remark1号"},

14             new Config{ ID=Guid.NewGuid(), Name="XX2号",Remark="Remark2号"},

15             new Config{ ID=Guid.NewGuid(), Name="XX3号",Remark="Remark3号"}

16         };

17     }

18 }

19 

20 

21 static void Linq3()

22 {

23     var source = Config.GetConfigs();

24     var query = from item in source

25                 select item.Name;

26     foreach (var c in query)

27     {

28         Console.WriteLine(c);

29     }

30 }

View Code

例3：数组下标转换

 1 static void Linq4()

 2 {

 3     int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };

 4     string[] strings = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

 5 

 6     var textNums =

 7         from n in numbers

 8         select strings[n];

 9 

10     Console.WriteLine("数组下标转换");

11     foreach (var s in textNums)

12     {

13         Console.WriteLine(s);

14     }

15 }

View Code

例3：匿名类型，大小写

 1 static void Linq5()

 2 {

 3     string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };

 4     var query = from word in words

 5                             select new { Upper = word.ToUpper(), Lower = word.ToLower() };

 6     foreach (var item in query)

 7     {

 8         Console.WriteLine("大写: {0} 小写: {1}",item.Upper,item.Lower);

 9     }

10 }

View Code

例4：将投影的属性放到一个类中

 1 public class Dto

 2 {

 3     public string Field1 { get; set; }

 4     public string Field2 { get; set; }

 5     public string Field3 { get; set; }

 6 }

 7 

 8 static void Linq6()

 9 {

10     var source = Config.GetConfigs();

11     var query = from item in source

12                 select new Dto { Field1 = item.ID.ToString(), Field2 = item.Name, Field3 = item.Remark };

13     foreach (var item in query)

14     {

15         Console.WriteLine("{0} {1} {2}", item.Field1, item.Field2, item.Field3);

16     }

17 }

View Code

例5：筛选 加where条件参见 where 例1

例6：复合from子句（选取座机号码区号为021的客户信息）

 1 public class Guest

 2 {

 3     /// <summary>

 4     /// 姓名

 5     /// </summary>

 6     public string Name { get; set; }

 7 

 8     /// <summary>

 9     /// 年龄

10     /// </summary>

11     public int Age { get; set; }

12 

13     /// <summary>

14     /// 电话表

15     /// </summary>

16     public List<string> TelTable { get; set; }

17 }

18 

19 

20 static void Linq8()

21 {

22     var source = new List<Guest>

23     {

24         new Guest{ Name="张三",Age=21,TelTable=new List<string>{"010-123456","13000000"}},

25         new Guest{ Name="李四",Age=22,TelTable=new List<string>{"021-123456","13100000"}},

26         new Guest{ Name="王五",Age=23,TelTable=new List<string>{"022-123456","13200000"}}

27     };

28     // 找电话簿中是021区号的客户

29     var query = from guest in source

30                 from tel in guest.TelTable

31                 where tel.Contains("021-")

32                 select guest;

33 

34     foreach (var guest in query)

35     {

36         Console.WriteLine("{0} 年龄：{1}", guest.Name, guest.Age);

37         foreach (var tel in guest.TelTable) { Console.WriteLine("     电话：{0}",tel); }

38     }

39 }

View Code

例7：多个 form 子句（交叉连接）

 1 static void Linq7()

 2 {

 3     int[] numbersA = { 0, 2, 4, 5, 6, 8, 9 };

 4     int[] numbersB = { 1, 3, 5, 7, 8 };

 5 

 6     var pairs =

 7         from a in numbersA

 8         from b in numbersB

 9         where a < b

10         select new { a, b };

11 

12     Console.WriteLine("交叉连接 a < b:");

13     foreach (var pair in pairs)

14     {

15         Console.WriteLine("{0} 小于 {1}", pair.a, pair.b);

16     }

17 }

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三、排序

orderby 排序字段

orderby 排序字段 descending

Thenby(orderby 多个排序字段)

ThenByDescending  (orderby 多个排序字段 descending)

例1：orderby 排序字段

 1 static void Linq9()

 2 {

 3     string[] words = { "cherry", "apple", "blueberry" };

 4 

 5     var sortedWords =

 6         from w in words

 7         orderby w

 8         select w;

 9 

10     Console.WriteLine("单词排序 order by 单个排序字段:");

11     foreach (var w in sortedWords)

12     {

13         Console.WriteLine(w);

14     }

15 }

View Code

例2：orderby 排序字段 descending

 1 static void Linq10()

 2 {

 3     string[] words = { "cherry", "apple", "blueberry" };

 4 

 5     var sortedWords =

 6         from w in words

 7         orderby w descending

 8         select w;

 9 

10     Console.WriteLine("单词排序 order by 单个排序字段 从大到小:");

11     foreach (var w in sortedWords)

12     {

13         Console.WriteLine(w);

14     }

15 }

View Code

例3：Thenby(orderby 多个排序字段)

 1 static void Linq11()

 2 {

 3     string[] words = { "cherry", "appppp", "cherryo", "appppppple", "blueberry" };

 4 

 5     // 先按名称排，再按长度排序

 6     var sortedWords =

 7         from w in words

 8         orderby w,w.Length

 9         select w;

10 

11     Console.WriteLine("单词排序 ThenBy 多个排序字段");

12     foreach (var w in sortedWords)

13     {

14         Console.WriteLine(w);

15     }

16 }

View Code

例4：ThenByDescending  (orderby 多个排序字段 descending)

注意主要条件和次要的升降序不传递

 1 static void Linq12()

 2         {

 3             string[] words = { "cherry", "appppp", "cherryo", "appppppple", "blueberry" };

 4 

 5             // 先按名称排，再按长度排序

 6             var sortedWords =

 7                 from w in words

 8                 orderby w descending, w.Length descending

 9                 select w;

10 

11             Console.WriteLine("单词排序 ThenBy 多个排序字段");

12             foreach (var w in sortedWords)

13             {

14                 Console.WriteLine(w);

15             }

16         }

View Code

 

 

本文代码： http://pan.baidu.com/s/1dDnJ0tV