IQueryable和IEnumerable的区别

IEnumerable接口

公开枚举器，该枚举器支持在指定类型的集合上进行简单迭代。也就是说：实现了此接口的object，就可以直接使用foreach遍历此object；

IQueryable 接口

它继承 IEnumerable 接口，而因为.net版本加入Linq和IQueryable后，使得IEnumerable不再那么单调，变得更加强大和丰富。

为了区别两个接口，我们通过一个实际的例子来解释一下。

[TestMethod]
       public void LinqTest()
       {
    
           var db = new SecurityRepositoryContext(new HSLogger());
           var ur = new UserRepository(db);
           //查询的结果放入IQueryable接口的集合中
           IQueryable<User> querys = (from c in db.Context.Set<User>()
                                      orderby c.Code
                                      select c).Skip<User>(2).Take<User>(2);
           //注意这个AsEnumerable<User>()在分页查询之前，先将其转换成IEnumerable类型
           IEnumerable<User> enumers = (from c in db.Context.Set<User>()
                                        orderby c.Code
                                        select c).AsEnumerable<User>().Skip<User>(2).Take<User>(2);
           //因为启用了延迟加载机制，所以下面调用一下，才会真正去读取数据库
           foreach (var c in querys)
           {
    
               //TODO
           }
           foreach (var c in enumers)
           {
     
               //TODO
           }
       }

跟踪一下代码，看一下这两段在执行时所生成的SQL代码：
第一段，返回IQueryable接口类型：

SELECT
    [Extent1].[Id] AS [Id], 
    [Extent1].[Code] AS [Code], 
    [Extent1].[Name] AS [Name], 
    [Extent1].[Status] AS [Status], 
    [Extent1].[IsOnline] AS [IsOnline], 
    [Extent1].[SystemVersion] AS [SystemVersion], 
    [Extent1].[CreatedBy] AS [CreatedBy], 
    [Extent1].[CreatedOn] AS [CreatedOn], 
    [Extent1].[ModifiedBy] AS [ModifiedBy], 
    [Extent1].[ModifiedOn] AS [ModifiedOn], 
    [Extent1].[VersionNumber] AS [VersionNumber]
    FROM [dbo].[User] AS [Extent1]
    ORDER BY [Extent1].[Code] ASC
    OFFSET 2 ROWS FETCH NEXT 2 ROWS ONLY

第二段，返回IEnumerable接口类型：

SELECT
    [Extent1].[Id] AS [Id], 
    [Extent1].[Code] AS [Code], 
    [Extent1].[Name] AS [Name], 
    [Extent1].[Status] AS [Status], 
    [Extent1].[IsOnline] AS [IsOnline], 
    [Extent1].[SystemVersion] AS [SystemVersion], 
    [Extent1].[CreatedBy] AS [CreatedBy], 
    [Extent1].[CreatedOn] AS [CreatedOn], 
    [Extent1].[ModifiedBy] AS [ModifiedBy], 
    [Extent1].[ModifiedOn] AS [ModifiedOn], 
    [Extent1].[VersionNumber] AS [VersionNumber]
    FROM [dbo].[User] AS [Extent1]
    ORDER BY [Extent1].[Code] ASC

通过上面两个例子我总结了一下，IQueryable接口与IEnumberable接口的区别：

IEnumerable 泛型类在调用自己的SKip 和 Take 等扩展方法之前数据就已经加载在本地内存里了，而IQueryable 是将Skip ,take 这些方法表达式翻译成T-SQL语句之后再向SQL服务器发送命令，它并不是把所有数据都加载到内存里来才进行条件过滤。