C#基础知识点总结（九）- LINQ

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已于 2023-05-05 11:01:53 修改

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分类专栏： C#基础知识点文章标签： linq c#

于 2022-10-29 19:21:35 首次发布

本文链接：https://blog.youkuaiyun.com/weixin_43152495/article/details/127589401

LINQ(Language Intergrated Query，语言集成查询)在C#编程语言中集成了查询语法，可以用相同的语法访问不同的数据源

1. LINQ概述

1.1 LINQ查询

1. 查询表达式必须以from子句开头，以select或group子句结束，在这两个子句之间，可以使用where，orderby，join，let和其他from子句

            var query = from r in Racer.ToListRacersInfo()
                        where r.name == "aaa"
                        orderby r.age descending
                        select r;

1.2 扩展方法

使用Enumerable类中的扩展方法Where(), OrderByDescending()和Select()。这些方法都返回IEnumerable<Source>

            var cham = new List<Racer>();
            IEnumerable<Racer> brazcham = cham.Where(r =< r.name == "aaa").
                                               OrderByDescending(r =< r.age).
                                               Select(r =< r);

1.3 推迟查询的执行

2. 标准的查询操作符

下面查询要用的数据源：

   //定义一个Student类
    public class Student
    {
        public int Id;
        public string Name;
        public string Gender;
        public int Age;
    }
    //定义一个 Person类
   public class Person
    {
        public int Num;
        public double salary;
        public string address;
        public int worktime;
    }

            List<Student> students = new List<Student>();
            students.Add(new Student() { Id = 1, Name = "张三", Gender = "男", Age = 23 });
            students.Add(new Student() { Id = 2, Name = "李四", Gender = "女", Age = 27 });
            students.Add(new Student() { Id = 3, Name = "王五", Gender = "男", Age = 21 });
            students.Add(new Student() { Id = 4, Name = "赵六", Gender = "女", Age = 25 });
            students.Add(new Student() { Id = 3, Name = "老王", Gender = "男", Age = 21 });
            students.Add(new Student() { Id = 4, Name = "麻子", Gender = "女", Age = 25 });

            List<Person> person = new List<Person>();
            person.Add(new Person() { Num = 1, salary = 1000, address = "番豆花园", worktime = 1 });
            person.Add(new Person() { Num = 1, salary = 2000, address = "番豆大街", worktime = 2 });
            person.Add(new Person() { Num = 3, salary = 3000, address = "番豆别墅",