IQueryable 和 IEnumerable

楼主最近看了下

IQueryable 和 IEnumerable

的区别。

当真被忽悠的死去活来。。。

网上都说 IQueryable 和 IEnumerable区别很大，然后怎么着怎么着。。。

然后我就去测试了啊

先拿出我的数据库表结构。

这里是我的查询语句。比较简单

TestDataEntities db = new TestDataEntities();
            IQueryable<Orders> q = db.Orders.OrderBy(x=>x.id).Skip(1).Take(2);
            IEnumerable<Orders> e = db.Orders.OrderBy(x=>x.id).Skip(1).Take(2);

            Console.WriteLine("IQueryable : \r\n"+q.ToString());
            Console.WriteLine("---------------------------------------------------------");
            Console.WriteLine("IEnumerable : \r\n" + e.ToString());

输出结果：

IQueryable :

SELECT TOP (2) 
[Extent1].[id] AS [id], 
[Extent1].[parentid] AS [parentid], 
[Extent1].[code] AS [code]
FROM ( SELECT [Extent1].[id] AS [id], [Extent1].[parentid] AS [parentid], [Extent1].[code] AS [code], row_number() OVER (ORDER BY [Extent1].[id] ASC) AS [row_number]
	FROM [dbo].[Orders] AS [Extent1]
)  AS [Extent1]
WHERE [Extent1].[row_number] > 1
ORDER BY [Extent1].[id] ASC

IEnumerable :

SELECT TOP (2) 
[Extent1].[id] AS [id], 
[Extent1].[parentid] AS [parentid], 
[Extent1].[code] AS [code]
FROM ( SELECT [Extent1].[id] AS [id], [Extent1].[parentid] AS [parentid], [Extent1].[code] AS [code], row_number() OVER (ORDER BY [Extent1].[id] ASC) AS [row_number]
	FROM [dbo].[Orders] AS [Extent1]
)  AS [Extent1]
WHERE [Extent1].[row_number] > 1
ORDER BY [Extent1].[id] ASC

有区别吗？没有哇！！！

后来楼主就较真了！！！！！！是的，较真了！！！为何网上这么多人说有区别！

然后发现。。。。是这个原因！~

看楼下代码

TestDataEntities db = new TestDataEntities();
            IQueryable<Orders> q = db.Orders.OrderBy(x => x.id).AsQueryable<Orders>().Skip(10).Take(2);
            IEnumerable<Orders> e = db.Orders.OrderBy(x => x.id).AsEnumerable<Orders>().Skip(10).Take(2);

            foreach (var item in q)
            {
                Console.WriteLine("123");
            }

            foreach (var item in e)
            {
                Console.WriteLine("234");
            }

原因其实出在

AsQueryable 和 AsEnumerable  

反正就是加载方式不同

上面代码的sql语句是这样的！

IQueryable :

SELECT TOP (2) 
[Extent1].[id] AS [id], 
[Extent1].[parentid] AS [parentid], 
[Extent1].[code] AS [code]
FROM ( SELECT [Extent1].[id] AS [id], [Extent1].[parentid] AS [parentid], [Extent1].[code] AS [code], row_number() OVER (ORDER BY [Extent1].[id] ASC) AS [row_number]
FROM [dbo].[Orders] AS [Extent1]
)  AS [Extent1]
WHERE [Extent1].[row_number] > 10
ORDER BY [Extent1].[id] ASC

IEnumerable :

SELECT 
[Extent1].[id] AS [id], 
[Extent1].[parentid] AS [parentid], 
[Extent1].[code] AS [code]
FROM [dbo].[Orders] AS [Extent1]
ORDER BY [Extent1].[id] ASC

是的，你没看错，奇怪在于IEnumerable生成的语句，是查询全表。

聪明的你肯定会联想到。IEnumerable是通过缓存全表，然后才分页查找的！！！所以IEnumerable性能不好！

为什么？？？？！！！！

因为：

IQueryable     是 linq to sql           这家伙操作数据库

IEnumerable 是  linq to object     这家伙操作内存

区别在这里！

当然网上还有说IQueryable 和 IEnumerable其他区别（这个楼主没有验证）

IEnumerable<T>查询必须在本地执行.并且执行查询前我们必须把所有的数据加载到本地

总结一下：

IQueryable     是 linq to sql         这家伙操作数据库

IEnumerable 是  linq to object     这家伙操作内存

IEnumerable 会查询全部数据，然后在内存里进行分页或者筛选操作。

但是真正导致他们使用哪种方式的，是AsQueryable() 和 AsEnumerable()  ！！！！！