标准查询运算符（SQO）

标准查询运算符定义在 System.Linq.Enumerable 类的 50 多个为 IEnumerable<T> 准备的扩展方法，换句话说，IEnumerable<T> 上的每一个方法都是一个标准查询运算符，这些方法用来对操作的集合进行查询筛选。

标准查询运算符提供了包括筛选、投影、聚合、排序等功能在内的查询功能。

实例如下：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;namespace ConsoleApplication1
{class Program{public static List<User> InitListData(){return new List<User>(){new User { Id = 1, Name = "凌霜残雪1", Age = 21 },new User { Id = 2, Name = "凌霜残雪2", Age = 22 },new User { Id = 3, Name = "凌霜残雪3", Age = 23 },new User { Id = 4, Name = "凌霜残雪4", Age = 24 },new User { Id = 5, Name = "凌霜残雪5", Age = 25 },new User { Id = 6, Name = "凌霜残雪6", Age = 26 },new User { Id = 7, Name = "凌霜残雪7", Age = 27 },new User { Id = 8, Name = "凌霜残雪8", Age = 28 },new User { Id = 9, Name = "凌霜残雪", Age = 29 },new User { Id = 10, Name = "凌霜残雪10", Age = 36 },new User { Id = 11, Name = "凌霜残雪11", Age = 33 }};}static void Main(string[] args){var list = InitListData();//var result = list.Where(x => x.Age >= 30).ToList();//result.ForEach(r => Console.WriteLine(string.Format("{0},{1},{2}", r.Id, r.Name, r.Age)));var result = list.Where(x => x.Age >= 30).Select(s => s.Name).ToList();result.ForEach(x => Console.WriteLine(x));Console.ReadLine();}}class User{public int Id { get; set; }public string Name { get; set; }public int Age { get; set; }}
}

筛选集合 Where

Where 方法为我们提供了一个集合的筛选功能，但需要一个带 bool 返回值的“筛选器”（匿名方法、委托、Lambda表达式均可），从而表明集合中某个元素是否应该被返回。

var list = InitListData();
var result = list.Where(x => x.Age >= 30).ToList();
result.Foreach(r=>Console.WriteLine(string.Format("{0},{1},{2}", r.Id, r.Name, r.Age)))
Console.ReadLine();

运行结果:

查询投射 Select:

返回对象集合 IEnumerable<T>Select()。返回年龄大于30 岁的人的名字代码如下：

 var result = list.Where(x => x.Age >= 30).Select(s => s.Name).ToList();result.ForEach(x => Console.WriteLine(x));

运行结果如下：

统计数量 int Count()

 int num = list.Where(x => x.Age >= 30).Count();

结果为 2.

多条件排序：OrderBy().ThenBy().ThenBy()

list.OrderBy(x=>x.Age).OrderBy(x=>x.Id)

集合连接 Join（）

新建一个 Student 类，并初始化数据。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;namespace ConsoleApplication1
{class Program{public static List<User> InitListData(){return new List<User>(){new User { Id = 1, Name = "凌霜残雪1", Age = 21 },new User { Id = 2, Name = "凌霜残雪2", Age = 22 },new User { Id = 3, Name = "凌霜残雪3", Age = 23 },new User { Id = 4, Name = "凌霜残雪4", Age = 24 },new User { Id = 5, Name = "凌霜残雪5", Age = 25 },new User { Id = 6, Name = "凌霜残雪6", Age = 26 },new User { Id = 7, Name = "凌霜残雪7", Age = 27 },new User { Id = 8, Name = "凌霜残雪8", Age = 28 },new User { Id = 9, Name = "凌霜残雪", Age = 29 },new User { Id = 10, Name = "凌霜残雪10", Age = 36 },new User { Id = 11, Name = "凌霜残雪11", Age = 33 }};}static void Main(string[] args){List<Student> lstStu = new List<Student>(){new Student { Id = 1, userID = 1, ClassName = "一班" },new Student { Id = 2, userID = 2, ClassName =  "一班" },new Student { Id = 3, userID = 3, ClassName =  "一班" },new Student { Id = 4, userID = 4, ClassName =  "一班" },new Student { Id = 5, userID = 5, ClassName =  "一班" },new Student { Id = 6, userID = 6, ClassName =  "一班" },new Student { Id = 7, userID = 7, ClassName =  "一班" },new Student { Id = 8, userID = 8, ClassName =  "一班" },new Student { Id = 9, userID = 9, ClassName =  "一班" },new Student { Id = 10, userID = 10, ClassName =  "一班" },new Student { Id = 11, userID = 11, ClassName =  "一班" }};var list = InitListData();//var result = list.Where(x => x.Age >= 30).ToList();//result.ForEach(r => Console.WriteLine(string.Format("{0},{1},{2}", r.Id, r.Name, r.Age)));//var result = list.Where(x => x.Age >= 30).Select(s => s.Name).ToList();//result.ForEach(x => Console.WriteLine(x));int num = list.Where(x => x.Age >= 30).Count();var result1 = list.Join(lstStu, u => u.Id, p => p.userID, (u, p) => new { userID = u.Id, Name = u.Name, ClassName = p.ClassName });foreach (var item in result1){Console.WriteLine(item);}Console.WriteLine(num);Console.ReadLine();}}class User{public int Id { get; set; }public string Name { get; set; }public int Age { get; set; }}class Student{public int Id { get; set; }public int userID { get; set; }public string ClassName { get; set; }}
}

运行结果如下：

标准查询运算符（SQO）相关推荐

.NET中那些所谓的新语法之四：标准查询运算符与LINQ
开篇:在上一篇中,我们了解了预定义委托与Lambda表达式等所谓的新语法,这一篇我们继续征程,看看标准查询运算符和LINQ.标准查询运算符是定义在System.Linq.Enumerable类中的50 ...
LINQ标准查询操作符
本文基于.NET Framework 2.0编写本文PDF下载推荐大家下载本文的PDF进行阅读,可以方便的使用书签来阅读各个方法,而且代码中的关键字是高亮显示的. 转载请注明出处. 一.投影操作符 ...
NHibernate3剖析：Query篇之NHibernate.Linq标准查询
本节内容系列引入 NHibernate.Linq概述标准查询运算符结语系列引入 NHibernate3.0剖析系列分别从Configuration篇.Mapping篇.Query篇.Sessi ...
一起谈.NET技术，NHibernate3.0剖析：Query篇之NHibernate.Linq标准查询
系列引入 NHibernate3.0剖析系列分别从Configuration篇.Mapping篇.Query篇.Session策略篇.应用篇等方面全面揭示NHibernate3.0新特性和应用及其各种 ...
LINQ：进阶 - LINQ 标准查询操作概述
"标准查询运算符"是组成语言集成查询 (LINQ) 模式的方法.大多数这些方法都在序列上运行,其中的序列是一个对象,其类型实现了IEnumerable<T> 接口或 I ...
NHibernate3.0剖析：Query篇之NHibernate.Linq标准查询
系列引入 NHibernate3.0剖析系列分别从Configuration篇.Mapping篇.Query篇.Session策略篇.应用篇等方面全面揭示NHibernate3.0新特性和应用及其各种 ...
标准查询分析器：Solr的默认查询解析器也称为“Lucene”解析器。
标准查询分析器:Solr的默认查询解析器也称为"Lucene"解析器. 标准查询解析器的主要优点是,它支持一种健壮且相当直观的语法,允许您创建各种结构化查询.最大的缺点是,与设计为 ...
C 标准转换运算符const_cast
C++标准转换运算符const_cast 前面讲了C++继承并扩展C语言的传统类型转换方式,最后留下了一些关于指针和引用上的转换问题,没有做详细地讲述.C++相比于C是一门面向对象的语言,面向对象最大 ...
C++标准转换运算符static_cast
C++标准转换运算符static_cast static_cast <new_type> (expression) 虽然const_cast是用来去除变量的const限定,但是static ...

标准查询运算符（SQO）

标准查询运算符（SQO）相关推荐

最新文章

热门文章