C# LINQ

一、概述

语言集成查询（Language Integrated Query），查询操作数据的工具。

二、使用方式

1、投射

投射是一种查询操作，从数据源中选择、变换或映射出出新的集合。Select来进行投射。投射一般会创建一个新的集合，而不会修改原数据。3

例：对订单列表进行投射，并且对价格打八折

using System.Net;
using System.Threading.Tasks;
using System.Linq;
class Program
{
    static readonly object _sync = new object();
    struct Order
    {
        public int id;
        public string name;
        public float price;
    };

    static void Main()
    {
        List<Order> orders = new List<Order>(){
            new Order(){id =1,name = "Order1",price=100},
            new Order(){id =2,name = "Order2",price=200},
            new Order(){id =3,name = "Order3",price=300}
            };
        var sale = orders.Select((order) => new { id = order.id, name = order.name, price = order.price * 0.8f });
        foreach (var item in sale)
        {
            Console.WriteLine(string.Format("{0},{1},{2}", item.id, item.name, item.price));
        }

    }
}

2、查询

例：查询订单号为2的订单

using System.Net;
using System.Threading.Tasks;
using System.Linq;
class Program
{
    static readonly object _sync = new object();
    struct Order
    {
        public int id;
        public string name;
        public float price;
    };

    static void Main()
    {
        List<Order> orders = new List<Order>(){
            new Order(){id =1,name = "Order1",price=100},
            new Order(){id =2,name = "Order2",price=200},
            new Order(){id =3,name = "Order3",price=300}
            };
        var sale = orders.Where((order) => order.id == 2);
        foreach (var item in sale)
        {
            Console.WriteLine(string.Format("{0},{1},{2}", item.id, item.name, item.price));
        }

    }
}

3、排序

OrderBy 和 OrderByDescending，分别是顺序和逆序。ThenBy 和 ThenByDescending可以进行级排序。

例：按照订单顺序进行正向排序。

using System.Net;
using System.Threading.Tasks;
using System.Linq;
class Program
{
    static readonly object _sync = new object();
    struct Order
    {
        public int id;
        public string name;
        public float price;
    };

    static void Main()
    {
        List<Order> orders = new List<Order>(){
            new Order(){id =1,name = "Order1",price=100},
            new Order(){id =3,name = "Order3",price=300},
            new Order(){id =2,name = "Order2",price=200}
            };
        var sale = orders.OrderBy((order) => order.id);
        foreach (var item in sale)
        {
            Console.WriteLine(string.Format("{0},{1},{2}", item.id, item.name, item.price));
        }
    }
}

4、分组

对数据根据类型进行分组

例：根据订单类型进行排序

using System.Net;
using System.Threading.Tasks;
using System.Linq;
class Program
{
    static readonly object _sync = new object();
    struct Order
    {
        public int id;
        public string name;
        public float price;
        public int orderType;
    };

    static void Main()
    {
        List<Order> orders = new List<Order>(){
            new Order(){id =1,name = "Order1",price=100,orderType = 1},
            new Order(){id =3,name = "Order3",price=300,orderType = 1},
            new Order(){id =2,name = "Order2",price=200,orderType = 2}
            };
        var orderGroups = from order in orders group order by order.orderType;
        foreach (var group in orderGroups)
        {
            foreach (var order in group)
            {
                Console.WriteLine("{0},{1},{2}", order.id, order.name, order.price);
            }
            Console.WriteLine("--------");
        }
    }
}