C# LINQ

LINQ

LINQ关键字

from 指定数据源和范围变量

where 根据布尔表达式(由逻辑与 或 等组成)从数据源中筛选元素

select 指定查询结果中的元素所具有的类型或表现形式

group 对对查询结果按照键值进行分组

into 提供一个标示符，它可以充当对 join group 或 select 子句结果的引用

orderby 对查询出的元素进行排序

join 按照两个指定匹配条件来联接俩个数据源

let 产生一个用于查询表达式中子表达式查询结果的范围变量

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

namespace ConsoleApplication7LINQ
{
    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()//重写ToString(),默认的ToString()仅输出类型名称
        {
            return "ID:" + ID + "City:" + City + "Country:" + Country + "Region:" + Region + "Sales:" + Sales;
        }
    }

    class Order
    {
        public string ID { get; set; }
        public decimal Amount { get; set; }
    }

    class Program
    {
        /// <summary>
        /// 生成随机数组
        /// </summary>
        /// <param name="count"></param>
        /// <returns></returns>
        private static int[] GenerateLotsofNumbers(int count)
        {
            Random generator = new Random(1); //使用指定的种子值初始化 Random 类的新实例。
            int[] result = new int[count];
            for (int i = 0; i < count; i++)
            {
                result[i] = generator.Next();
            }
            return result;
        }

        //1.用var关键字声明结果变量
        //2.指定数据源：from 子句
        //3.指定条件：where 子句
        //4.指定元素：select子句
        //5.完成：使用foreach循环
        static void Main(string[] args)
        {
           // string[] names = { "Alono", "Zheng", "Yuan", "Song", "Simle", "Hsieh", "Small", "She", "Sza", "Sam", "Fa", "Iyl" };
           // //var queryResults = from n in names
           // //                   where n.StartsWith("S")
           // //                   orderby n descending//按照最后一个字母排序 ordeby n.Substring(n.Length - 1)
           // //                   select n;//查询语法
           // var queryResults = names.OrderBy(n => n).Where(n => n.StartsWith("S"));//方法语法 Lambda 表达式
           //// var queryResults = names.OrderByDescending(n => n).Where(n => n.StartsWith("S"));
           // foreach (var item in queryResults)
           //     Console.WriteLine(item);
           // Console.ReadKey();

           //--------------------------------------------------------------------------------------------------

            //int[] numbers = GenerateLotsofNumbers(12345678);
            //var queryResults = from n in numbers
            //                   where n < 1000
            //                   select n;
            //foreach (var item in queryResults)
            //{
            //    Console.WriteLine(item);
            //}
            //Console.WriteLine("聚合运算符......");
            //Console.WriteLine(queryResults.Count());
            //Console.WriteLine(queryResults .Max ());
            //Console.WriteLine(queryResults.Average());
            //Console.WriteLine(queryResults.Sum());
            //Console.ReadKey();

            //--------------------------------------------------------------------------------------------------

            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 ="New York",Country ="USA",Region ="North America",Sales =9999},
                 new Customer {ID ="C",City ="XiAn",Country ="China",Region ="Asia",Sales =7777},
                  new Customer {ID ="D",City ="New York",Country ="USA",Region ="North America",Sales =9999},
                   new Customer {ID ="E",City ="BeiJing",Country ="China",Region ="Asia",Sales =8888},
                    new Customer {ID ="F",City ="New York",Country ="USA",Region ="North America",Sales =9999}
            };
            //var queryReaults =
            //    from n in customers
            //    where n.Region == "Asia"
            //    select n;
            //foreach (var item in queryReaults )
            //    Console.WriteLine(item);
            //Console.ReadKey();
            
            //--------------------------------------投影----------------------------------------------
            //投影是在LINQ查询中从其他数据类型中创建新数据类型的术语。
            //var queryResults =
            //    from c in customers
            //    where c.Region == "Asia"
            //    select new { c.City, c.Country, c.Sales };
            //var queryResults = customers.Where(c => c.Region == "Asia").Select(c => new { c.City, c.Country, c.Sales });
            //var queryResults = customers.Select(c => new { c.City, c.Country, c.Sales }).Where(c => c.City == "XiAn");
            //foreach (var item in queryResults)
            //    Console.WriteLine(item);
            //Console.ReadKey();
            
            //--------------------单值选择查询------------------------
            var queryResults1 = customers.Select(c => c.Region).Distinct();
            var queryResults2 = (from c in customers select c.Region).Distinct();

            //------------------------Any和All---------------------------------
            bool anyUSA = customers.Any(c => c.Country == "USA");
            if(anyUSA )
                Console.WriteLine("some customers are in USA");
            else
                Console.WriteLine("WAWA");
            bool allAsia = customers.All(c => c.Region == "Asia");
            if(allAsia )
                Console.WriteLine("WAWA");
            else
                Console.WriteLine("All customers are in Asia");

           // Console.ReadKey();

            //--------------------------------多级排序---------------------------------------
            var queryReaults3 =
                from n in customers
                where n.Region == "Asia"
                orderby n.Region ,n.Country descending ,n.City //查询语法 多级排序
                select n;
            var queryResults4 =
                customers.OrderBy(c => c.Region).ThenByDescending(c => c.Country).ThenBy(c => c.City).Select(c => new { c.ID, c.Region, c.Country, c.City });
                //方法语法 多级排序

            //------------------------------------组合查询（group query）-------------------------------------
            //组合查询中的数据通过一个键（Key）字段来组合，每一个组中的所有成员都共享这个字段值，在这个例子中 键字段是Region 
            //要计算每个组的总和，应先生成一个新的结果集cg
            var queryResults5 = from c in customers
                                group c by c.Region into cg
                                select new { TotalSales = cg.Sum(c => c.Sales), Region = cg.Key };
            var orderedResults = from cg in queryResults5
                                 orderby cg.TotalSales descending
                                 select cg;
            foreach (var item in orderedResults)
                Console.WriteLine(item.TotalSales + "	:" + item.Region);
            //Console.ReadKey();

            //------------------------Take 和 Skip----------------------------------
            //Take() 从查询结果中提取前n个结果
            //Skip() 从查询结果中跳过前n个结果 返回剩余的结果
            foreach (var item in orderedResults .Take (2))
                Console.WriteLine(item.TotalSales + "	:" + item.Region);

            //---------------------------First 和 FirstOrDefault-------------------------------
            //First() 返回结果集中第一个匹配给定条件的元素
            //FirstOrDefault() 当查询条件不满足是，将为列表返回默认元素 而使用First()则返回null
            Console.WriteLine(queryReaults3.FirstOrDefault(n => n.Region == "Asia"));

            //--------------------------------集运算符-----------------------------
            List<Order> orders = new List<Order>{
            new Order {ID="A",Amount=100},
            new Order {ID ="B",Amount =200},
            new Order {ID ="H",Amount =300}};

            var customersIDs = from c in customers
                               select c.ID;
            var ordersIDs = from o in orders
                            select o.ID;
            var customersWithOrders = customersIDs.Intersect(ordersIDs);//Intersect()
            foreach (var item in customersWithOrders )
                Console.WriteLine(item );
            Console.WriteLine("-------------------------");
            var ordersNoCustomers = ordersIDs.Except(customersIDs);//Except()
            foreach (var item in ordersNoCustomers )
                Console.WriteLine(item );
            Console.WriteLine("-------------------------");
            var allCustomersOrders = ordersIDs.Union(customersIDs);//Union()
            foreach (var item in allCustomersOrders)
                Console.WriteLine(item);
            //Console.ReadKey();

            //---------------------------------Join----------------------------
            //使用Join运算符在一个查询中查找多个集合中的相关数据，用键字段把结果连接起来
            var queryResults9 =
                from c in customers
                join o in orders on c.ID equals o.ID
                select new { c.ID, c.City, SalesBefore = c.Sales, NewOrder = o.Amount, SalesAfter = c.Sales + o.Amount };
            foreach (var item in queryResults9 )
                Console.WriteLine(item );
            Console.ReadKey();

        }
    }
}

into子句

into子句作为一个临时标识符，用于group select join 子句中。它存储了into子句前面的查询内容，是后面的子句可以方便的使用，对其进行再次查询或排序 投影等操作。

let子句

let语句在LINQ表达式中存储子表达式的计算结果。let子句创建一个范围变量来存储结果，变量被创建后，不能修改或把其他表达式的结果重新赋值给它。此范围变量可以在后续的LINQ语句中使用。

join子句

如果一个数据源中元素的某一个属性可以跟另外一个数据源中元素的属性进行相等比较，那么着两个数据源可以用join子句进行关联

join子句使用equals关键字进行相等比较

注：以上代码来自《C#入门经典5》《LINQ入门及应用》!!!