LINQ -2015-04-27

LINQ--language-integrated-query

1、它和sql语言区别呢？

SQL语言常用在ralational-database中，而LINQ对内存数据，数据库，xml文件等多种形式进行操作。简而言之，操作范围广。

2、下面我们简单地来实现一下这个基本的linq：

首先，创一个customer类：

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

namespace LINQ_Exam
{
    //create a simple Customer class
    public class Customer
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string HomeAddress{ get; set; }
        //now override the ToString function of Object class.
        public override string ToString()
        {
            return string.Format("{0} {1} Enmail:{2}",FirstName,LastName,HomeAddress);
        }
        public static List<Customer> CreateCustomerList()
        {
            List<Customer> customers = new List<Customer>
            {
                 new Customer { FirstName = "Orlando",LastName = "Gee", HomeAddress = "orlando0@adventure-works.com"},
                 new Customer { FirstName = "Keith", LastName = "Harris",HomeAddress = "keith0@adventure-works.com" },
                 new Customer { FirstName = "Donna", LastName = "Carreras",HomeAddress = "donna0@adventure-works.com" },
                 new Customer { FirstName = "Janet", LastName = "Gates",HomeAddress = "janet1@adventure-works.com" },
                 new Customer { FirstName = "Lucy", LastName = "Harrington",HomeAddress = "lucy0@adventure-works.com" }
            };
            return customers;
        }
    }
}

然后，写几句话去用LINQ了，我在test类中main函数中去实现：

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

namespace LINQ_Exam
{
    public class Tester
    {
        public static void Main()    
        {
            List<Customer> customers = Customer.CreateCustomerList();
            IEnumerable<Customer> result = from customer in customers
                                           where customer.FirstName == "Donna"
                                           select customer;
            Console.WriteLine("FirstName = "Donna"");
            foreach(Customer customer in result)
            {
                Console.WriteLine(customer.ToString());
            }
            customers[3].FirstName = "Donna";
             Console.WriteLine("FirstName == "Donna" (take two)");
             foreach (Customer customer in result)
                {  Console.WriteLine(customer.ToString());}
          }
       }
}
然后执行结果为：

可见，linq好似和sql的用法差不多，只是要记住其范围可用更加广阔。

3、在这小例子中，我们需要注意的问题：

*

from   customer  in customers  这里的customer其实是一个变量名称，它迭代了customers里面的每一个数值，就相当于吧customers里面的

每一个值去赋予到customer中，然后进行筛选，为真的则保留下来，然后得到最后的查找结果。

*

Deferred(延期的) Query Evaluation

IEnumerable<Customer> result = from   customer  in customers

  where  customer.FirstName == "Donna“   select  customer；

这个是定义一个查找结果的代码，当代码写出来的时候，代码并没有运行产生一个result，此时仅仅是定义一个result的变量，这个变量里还没有任何数据。

因为下面的LINQ语句并没有执行。然而多会执行了呢？-------------------当result需要被用到的时候，就被执行了，就相当于result现场被填充了内容。

如：

foreach (Customer customer in result) {………}这个语句用到了result，然后result就会被立马填充内容了

这个很类似于sql中的view（视图）视图是空的，显示视图的内容只是进行一个对表的查找，然后再显示出来。就是这样啦。

ps：如果你不想让它这么干，可以这么写:(用到了ToList或者ToArray)

ToList<Customer> result = from   customer  in customers

  where  customer.FirstName == "Donna“   select  customer；