C# LINQ语句

Lanuague Intergarted Query

语言整合查询

我们可以使用它来查询一些信息，如我们查询数据库一样。

使用System.Linq命名空间

参考：https://www.bilibili.com/video/BV1TJ411h7cZ?t=2&p=230

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;

namespace ConsoleApp4
{
    delegate void test(string s);
    class Program
    {
        static void Main(string[] args)
        {
            int[] numbers = {1, 3, 5, 16, 8, 1, 0, 5, 7};//实现了IEnumerable 的数据可以使用LINQ
            //1.直接使用语句
            var numQurey1 = from num in numbers
                            where num % 2 == 0 //查询条件
                            orderby num //排序
                            select num; //输出
            foreach(var i in numQurey1)//这个语句在使用到的时候才会被执行
            {
                Console.Write(i + " ");
            }
            //2.使用方法
            var numQurey2 = numbers.Where(n => n % 2 == 0).OrderBy(n => n);
            foreach (var i in numQurey2)//这个语句在使用到的时候才会被执行
            {
                Console.Write(i + " ");
            }
            //用到的查询关键字和方法还有很多
            //方法：numQurey2.Count() 获得长度 numQurey2.ToList() 转换为列表 
            //    numQurey2.ToArray() 转换为数组 这些函数执行时会立即执行语句

            //关键字：
            //where 使用 ||  && 等操作符来控制条件 如：x < 0 || x>5
            //orderby num descending 获得的结果降序排列 orderby num ascending 获得的结果降序排列
            //group 查询的结果分组
            List<Customer> customers = new List<Customer>();
            customers.Add(new Customer() { Name = "Jack", City = "Beijing" });
            customers.Add(new Customer() { Name = "LiLei", City = "Beijing" });
            customers.Add(new Customer() { Name = "WangMei", City = "Shanghai" });

            var queryCustomers = from c in customers
                                 group c by c.City;

            foreach(var i in queryCustomers)
            {
                Console.WriteLine();
                Console.WriteLine(i.Key);//Key 就是分组的键值
                foreach(var j in i)
                {
                    Console.WriteLine("  " + j.Name + "  " + j.City);
                }
            }

            //join 合并两个数据

            List<Employee> employees = new List<Employee>();
            employees.Add(new Employee() { Name = "Jack", ID = 101 });
            employees.Add(new Employee() { Name = "Emily", ID = 102 });

            var queryJoin = from c in customers
                            join e in employees on c.Name equals e.Name
                            select new { PersonName = c.Name, PersonID = e.ID, PerCity = c.City };

            foreach(var i in queryJoin)
            {
                Console.WriteLine();
                Console.WriteLine(i.PersonName + "  " + i.PersonID + "  " + i.PerCity);
            }

            //into group的结果进行打包
            var queryCustomers2 = from c in customers
                                  group c by c.City into customerGroup
                                  where customerGroup.Count() >= 2
                                  select new { city = customerGroup.Key, number = customerGroup.Count() };

            foreach(var i in queryCustomers2)
            {
                Console.WriteLine();
                Console.WriteLine(i.city + "  " + i.number);
            }

            //let 可以存储中间变量
            string[] strings = { "I am hansome", "Who are you", "Who I am" };
            var stringQuery = from s in strings
                              let words = s.Split()
                              from word in words
                              let w = word.ToUpper()
                              select w;
            Console.WriteLine();
            foreach (var i in stringQuery)
            {
                Console.WriteLine(i);
            }
        }
    }

    class Customer
    {
        public string Name
        {
            get;
            set;
        }

        public string City
        {
            get;
            set;
        }
    }
    class Employee
    {
        public string Name
        {
            get;
            set;
        }

        public int ID
        {
            get;
            set;
        }
    }
}

当然更多的用法需要百度自行查询。