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[Linq] LinqToObject

Technorati 标签: Linq

Linq 是C# 3.0 新增的好东西，用来处理对象集合，可用来查询数据库，XML文档

主要分为    Linq To Objects （用来处理内存对象集合），也是本节的重点

               Linq TO SQL

               Linq TO XML

               Lnq To Providers

1. 基本语法

var n=

from 数据源

where ..

select n;

最后 foreach( var a in n) 循环结果集，例子将展示最基本的用法.

例子namespace Linq_1
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] strNames = { "小明","小李","小王","大胡","大大"};
            
            var queryResults =
                from n in strNames
                where n.StartsWith("小")
                select n;

            Console.WriteLine("名子以小开头的有: ");

            foreach (var item in queryResults)
            {
                Console.WriteLine(item);

            }

            Console.ReadKey();
        }
    }
}

声明字符串数组 strNames；然后linq表达式 from 指定数据源,where 指明条件,select n 原则结果集

最后通过 foreach 获取结果集，延迟执行！将名字中以“小”开头的名字打印出来。

2. 用Linq方法语法和λ表达式（n=>xx）

1) λ表达式

n=>n<100 就是匿名函数，n为参数，n<100是函数或者方法体。若n<100则返回Ture

2)Linq方法语法

同样例子查找以“小”开头的人名

var queryResult=strNames.where(n=>n.startwith("小"))；

Where方法static void Main(string[] args)
        {
            string[] strNames = { "小明", "小李", "小王", "大胡", "大大" };

            var queryResults = strNames.Where(n => n.StartsWith("小"));


            Console.WriteLine("名子以小开头的有: ");

            foreach (var item in queryResults)
            {
                Console.WriteLine(item);

            }

            Console.ReadKey(); 
        }

方法：OrderBy

OrderBy方法var queryResults2 = strNames.OrderBy(n => n).Where(n => n.StartsWith("小"));
            
            foreach (var item in queryResults2)
            {
                Console.WriteLine(item);
            }

3. 合计运算符

Count(),Min(),Max(),Average(),Sum()

合计运算符 class Program
    {
        static void Main(string[] args)
        {
            int[] numbers = generateLotsOfNumbers(12345678);


            //小于1000的数字
            var queryResults =
                from n in numbers
                where n < 1000
                select n;


            Console.WriteLine("小于1000的数有：");

            foreach (var str in queryResults)
            {

                Console.WriteLine(str);

            }







            //合计运算符操作

            var queryResults2 =
                from n in numbers
                where n > 1000
                select n;

            Console.WriteLine("Count of numbers >1000 ");
            Console.WriteLine(queryResults2.Count());


            Console.WriteLine("min  of  numbers >1000");
            Console.WriteLine(queryResults2.Min());

            Console.WriteLine("max  of  numbers >1000");
            Console.WriteLine(queryResults2.Max());

            Console.WriteLine("average  of  numbers >1000");
            Console.WriteLine(queryResults2.Average());


            Console.WriteLine("sum  of  numbers >1000");
            Console.WriteLine(queryResults2.Sum(n=>(long)n));



            Console.ReadKey();
            
        }


        /// <summary>
        /// 生成一个随机数列表
        /// </summary>
        /// <param name="count"></param>
        /// <returns></returns>
        private static int[] generateLotsOfNumbers(int count)
        {
            Random generator = new Random(0);
            int[] result = new int[count];

            for (int i = 0; i < count; i++)
            {
                result[i] = generator.Next();
            }

            return result;
        }
    }

查询复杂对象using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Linq_查询复杂对象
{

    class Customer
    {
        
        public string ID { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Region {get;set; }
        public decimal Sales{get;set;}


        public override string ToString()
        {
            return "ID: " + ID + " City" + City + "  Country" + Country + " Region" + Region + " Sales" + Sales;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            //查询asia亚洲
            List<Customer> customers = new List<Customer>
            {
                new Customer{ID="A",City="New York",Country="USA",Region="North America",Sales=9999},
                 new Customer{ID="B",City="Mumbai",Country="India",Region="Asia",Sales=8888},
                  new Customer{ID="C",City="Karachi",Country="Pakistan",Region="Asia",Sales=7777},
            };


            var queryResults =
                from c in customers
                where c.Region == "Asia"
                select c;

            Console.WriteLine("Customers in Asia:");

            foreach (var a in queryResults)
            {
                Console.WriteLine(a);
            }












            Console.ReadKey();
        }
    }
}

4. 投影：查询中创建对象

    select n+1

    select n.ToUpper()

    错误：select n.name,n.city    只允许select one.所以当要查询多个数据时候应该创建对象

select new {n.name,n.city}

投射方法语法-在查询中创建对象using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

//投射.在查询中创建新对象
namespace Linq_投射.在查询中创建新对象
{

    class Customer
    {

        public string ID { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Region { get; set; }
        public decimal Sales { get; set; }


        public override string ToString()
        {
            return "ID: " + ID + " City" + City + "  Country" + Country + " Region" + Region + " Sales" + Sales;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            //查询asia亚洲
            List<Customer> customers = new List<Customer>
            {
                new Customer{ID="A",City="New York",Country="USA",Region="North America",Sales=9999},
                 new Customer{ID="B",City="Mumbai",Country="India",Region="Asia",Sales=8888},
                  new Customer{ID="C",City="Karachi",Country="Pakistan",Region="Asia",Sales=7777},
            };


            var queryResults =
                from c in customers
                where c.Region == "Asia"
                select new { c.City, c.Country, c.Sales };   //投射.在查询中创建新对象


            var queryResults2 = customers.Where(c => c.Region == "Asia").Select(c => new { c.City, c.Country, c.Sales });

            Console.WriteLine("Customers in Asia:");

            foreach (var a in queryResults)
            {
                Console.WriteLine(a);
            }




            Console.WriteLine("Customers in Asia:");

            foreach (var a in queryResults2)
            {
                Console.WriteLine(a);
            }









            Console.ReadKey();
        }
    }
}

5. 单值选项 select(n=>new { xx}).Distinct();

6. Any()和All() 查找数据源是否有，有就Ture，否则false

例：查找是否有集合项C的country 是否为 “USA”

bool anyUsa=Customers.Any(n=>n.country=="USA");

7. 多级排序 OrderBy(x).ThenBy(x)

多级排序using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;


//orderby(c=>xx).ThenBy(c=>c.xx)；    
// Take()类似SQL 的top,skip()跳过前几个结果; 
//First,firstorDefault()
namespace Linq__多级排序
{
    class Customer
    {

        public string ID { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Region { get; set; }
        public decimal Sales { get; set; }


        public override string ToString()
        {
            return "ID: " + ID + " City" + City + "  Country" + Country + " Region" + Region + " Sales" + Sales;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            //查询asia亚洲
            List<Customer> customers = new List<Customer>
            {
                new Customer{ID="A",City="New York",Country="USA",Region="North America",Sales=9999},
                 new Customer{ID="B",City="Mumbai",Country="India",Region="Asia",Sales=8888},
                  new Customer{ID="C",City="Karachi",Country="Pakistan",Region="Asia",Sales=7777},
            };



            //普通排序

            var queryResults =
                from c in customers
                orderby c.Country,c.ID,c.City
                select new { c.City, c.Country, c.Sales };   


           

            Console.WriteLine("Customers in Asia:");

            foreach (var a in queryResults)
            {
                Console.WriteLine(a);
            }




            //多级排序方法
            var queryResults2 = customers.OrderBy(c => c.Country).ThenBy(c => c.ID).ThenBy(c => c.Sales).Select(c => new { c.City, c.Country, c.Sales });


            foreach (var a in queryResults2)
            {
                Console.WriteLine(a);
            }





            //Take() 取前1个结果

            Console.WriteLine("queryResults2 的第一个结果是:");
            foreach (var a in queryResults2.Take(1))
            {
                Console.WriteLine(a);
            }




            //Skip(int) 排除前几个结果

            Console.WriteLine("queryResults2 出去第一个结果是:");
            foreach (var a in queryResults2.Skip(1))
            {
                Console.WriteLine(a);
            }




            //First()
            Console.WriteLine("queryResults2 coutry是USA 的第一个结果是:");
            Console.WriteLine(queryResults2.First(b=>b.Country=="USA"));


            Console.WriteLine("queryResults2 coutry是china 的第一个结果是:");
            Console.WriteLine(queryResults2.FirstOrDefault(b => b.Country == "china"));

            Console.ReadKey();
        }
    }
}

8. 组合查询

　　　　　　　 from c in xx

　　　　　　　　group c by c.Region into cq

                    select   new {

                                       TotalPrice=cq.Sum(n=>c.sales),

                                        Region=cq.key

                                      }

9. Take() 类似 MSSQL 的Top（），返回前n个结果

Skip（）跳过前n个结果集

Console.WriteLine("queryResults2 的第一个结果是:");
            foreach (var a in queryResults2.Take(1))
            {
                Console.WriteLine(a);
            }

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原文地址：https://www.cnblogs.com/StudyLife/p/2369621.html