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。
*
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；