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 = { , , , , , - };
var query = from n in numbers
            where n %  ==
            select n;
Console.WriteLine("被3整除的数：");
foreach (var num in numbers)
{
    Console.WriteLine(num);
}

多个Where 条件且筛选

 var query = from item in sources
             where (字段1的条件1) || (字段1的条件2)
             where (字段2的条件1) || (字段2的条件2)
             select new Entity ....

 如
 var query = from item in sources
             where (string.IsNullOrEmpty(txtSearch.Text.Trim())) || (item.Filed1.Contains(txtSearch.Text.Trim()))
             where (string.IsNullOrEmpty(txtFilter.Text.Trim())) || (item.Filed2.Contains(txtFilter.Text.Trim()))
             ...
             select new Entity ....

二、投影——select

例1：让数组中每个数都加1

 int[] numbers = { , , , , , ,- };

 var numsPlusOne =
     from n in numbers
     select n + ;

 Console.WriteLine("整数+1:");
 foreach (var num in numsPlusOne)
 {
     Console.WriteLine(num);
 }

例2：选取某类中一个属性

 public class Config
 {
     public Guid ID { get; set; }
     public string Name { get; set; }
     public string Remark { get; set; }

     Config() { }

     public static List<Config> GetConfigs()
     {
         return new List<Config>
         {
             new Config{ ID=Guid.NewGuid(), Name="XX1号",Remark="Remark1号"},
             new Config{ ID=Guid.NewGuid(), Name="XX2号",Remark="Remark2号"},
             new Config{ ID=Guid.NewGuid(), Name="XX3号",Remark="Remark3号"}
         };
     }
 }

 static void Linq3()
 {
     var source = Config.GetConfigs();
     var query = from item in source
                 select item.Name;
     foreach (var c in query)
     {
         Console.WriteLine(c);
     }
 }

例3：数组下标转换

 static void Linq4()
 {
     int[] numbers = { , , , , , , , , ,  };
     string[] strings = { "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };

     var textNums =
         from n in numbers
         select strings[n];

     Console.WriteLine("数组下标转换");
     foreach (var s in textNums)
     {
         Console.WriteLine(s);
     }
 }

例3：匿名类型，大小写

 static void Linq5()
 {
     string[] words = { "aPPLE", "BlUeBeRrY", "cHeRry" };
     var query = from word in words
                             select new { Upper = word.ToUpper(), Lower = word.ToLower() };
     foreach (var item in query)
     {
         Console.WriteLine("大写: {0} 小写: {1}",item.Upper,item.Lower);
     }
 }

例4：将投影的属性放到一个类中

 public class Dto
 {
     public string Field1 { get; set; }
     public string Field2 { get; set; }
     public string Field3 { get; set; }
 }

 static void Linq6()
 {
     var source = Config.GetConfigs();
     var query = from item in source
                 select new Dto { Field1 = item.ID.ToString(), Field2 = item.Name, Field3 = item.Remark };
     foreach (var item in query)
     {
         Console.WriteLine("{0} {1} {2}", item.Field1, item.Field2, item.Field3);
     }
 }

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）

 public class Guest
 {
     /// <summary>
     /// 姓名
     /// </summary>
     public string Name { get; set; }

     /// <summary>
     /// 年龄
     /// </summary>
     public int Age { get; set; }

     /// <summary>
     /// 电话表
     /// </summary>
     public List<string> TelTable { get; set; }
 }

 static void Linq8()
 {
     var source = new List<Guest>
     {
         new Guest{ Name="张三",Age=,TelTable=new List<string>{"010-123456",""}},
         new Guest{ Name="李四",Age=,TelTable=new List<string>{"021-123456",""}},
         new Guest{ Name="王五",Age=,TelTable=new List<string>{"022-123456",""}}
     };
     // 找电话簿中是021区号的客户
     var query = from guest in source
                 from tel in guest.TelTable
                 where tel.Contains("021-")
                 select guest;

     foreach (var guest in query)
     {
         Console.WriteLine("{0} 年龄：{1}", guest.Name, guest.Age);
         foreach (var tel in guest.TelTable) { Console.WriteLine("     电话：{0}",tel); }
     }
 }

例7：多个 form 子句（交叉连接）

 static void Linq7()
 {
     int[] numbersA = { , , , , , ,  };
     int[] numbersB = { , , , ,  };

     var pairs =
         from a in numbersA
         from b in numbersB
         where a < b
         select new { a, b };

     Console.WriteLine("交叉连接 a < b:");
     foreach (var pair in pairs)
     {
         Console.WriteLine("{0} 小于 {1}", pair.a, pair.b);
     }
 }

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

 static void Linq9()
 {
     string[] words = { "cherry", "apple", "blueberry" };

     var sortedWords =
         from w in words
         orderby w
         select w;

     Console.WriteLine("单词排序 order by 单个排序字段:");
     foreach (var w in sortedWords)
     {
         Console.WriteLine(w);
     }
 }

例2：orderby 排序字段 descending

 static void Linq10()
 {
     string[] words = { "cherry", "apple", "blueberry" };

     var sortedWords =
         from w in words
         orderby w descending
         select w;

     Console.WriteLine("单词排序 order by 单个排序字段 从大到小:");
     foreach (var w in sortedWords)
     {
         Console.WriteLine(w);
     }
 }

例3：Thenby(orderby 多个排序字段)

 static void Linq11()
 {
     string[] words = { "cherry", "appppp", "cherryo", "appppppple", "blueberry" };

     // 先按名称排，再按长度排序
     var sortedWords =
         from w in words
         orderby w,w.Length
         select w;

     Console.WriteLine("单词排序 ThenBy 多个排序字段");
     foreach (var w in sortedWords)
     {
         Console.WriteLine(w);
     }
 }

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

 static void Linq12()
         {
             string[] words = { "cherry", "appppp", "cherryo", "appppppple", "blueberry" };

             // 先按名称排，再按长度排序
             var sortedWords =
                 from w in words
                 orderby w descending, w.Length descending
                 select w;

             Console.WriteLine("单词排序 ThenBy 多个排序字段");
             foreach (var w in sortedWords)
             {
                 Console.WriteLine(w);
             }
         }

本文代码： http://pan.baidu.com/s/1dDnJ0tV