Linq初级学习 [标准查询操作符的使用和调用]-优快云博客

本文链接：https://blog.youkuaiyun.com/neusoft06/article/details/8817772

本文详细介绍了LINQ中的投影操作符Select和SelectMany的用法，通过实例演示了如何使用这些操作符对集合中的值进行投影，从而实现从序列中返回指定的列。

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01 投影操作符(Select,SelectMany)

02 限制操作符(Where)

03 排序操作符(OrderBy,OrderByDescending,ThenBy,ThenByDescending,Reverse)

04 联接操作符(join,GroupJoin)

05 分组操作符(GroupBy)

06 串联操作符(Concat)

07 聚合操作符(Aggregate,Average,Count,LongCount,Max,Min,Sum)

08 集合操作符(Distinct,Union,Intersect,Except)

09 生成操作符(Empty,Range,Repeat)

10 转换操作符(Cast,OfType,ToArray,ToDictionary,ToList,ToLookup)

11 元素操作符(DefaultIfEmpty,ElementAt,ElementAtOrDefault,First,Last,FirstOrDefault,LastOrDefault,Single,SingleOrDefault)

12 相等操作符(SequenceEqual)

13 量词操作符(All,Any)

14 分割操作符(Skip,SkipWhile,Take,TakeWhile)

01.投影操作符(Select,SelectMany)

(1)Select操作符对单个序列或者集合中的值进行投影,可以控制从序列中返回指定的列.

方法语法:

 1 using System;
 2 using System.Collections.Generic;
 3 using System.Linq;
 4 using System.Text;
 5 using System.Data.Linq;
 6 
 7 namespace ConsoleApplication1
 8 {
 9     class Program 
10     {
11         static void Main(string[] args)
12         {
13             //创建测试数据    
14             User[] users = new User[]{
15                 new User(){ Id = 1, FirstName = "Huang", LastName = "Cong", Age = 12},
16                 new User(){ Id = 2, FirstName = "Yang", LastName = "Qiu", Age = 22},     
17                 new User(){ Id = 3, FirstName = "Cai", LastName = "Zheng", Age = 18},   
18                 new User(){ Id = 4, FirstName = "Huang", LastName = "Chao", Age = 32},   
19                 new User(){ Id = 5, FirstName = "Teng", LastName = "Jie", Age = 12},      
20                 new User(){ Id = 6, FirstName = "Liang", LastName = "Zhi", Age = 8}      
21             };     
22             //使用Select操作符进行投影,对序列只返回First一个列     
23             var query = users.Select(u => new { u.FirstName });   
24             //输出           
25             foreach (var q in query) 
26                 Console.WriteLine(q.FirstName); 
27         }   
28     }
29     class User
30     {
31         public int Id { get; set; } 
32         public string FirstName { get; set; }     
33         public string LastName { get; set; }   
34         public int Age { get; set; }    
35         public override string ToString()     
36         {         
37             return String.Format("Id:{0}\tFirstName:{1}\tLastName:{2}\tAge:{3}", this.Id, this.FirstName, this.LastName, this.Age);  
38         }
39     }
40 }

查询语法:

       select sname,ssex,
       class 
       from student
      
       Linq:
      
       from s 
       in 
       Students
      
       select 
       new 
       {
      
       s.SNAME,
      
       s.SSEX,
      
       s.CLASS
      
       }
      
       Lambda:
      
       Students.Select( s => 
       new 
       {
      
       SNAME = s.SNAME,SSEX = s.SSEX,CLASS = s.CLASS
      
       })