将不确定变为确定~DAL层向BLL层返回数据用IEnumerable<T>还是IQueryable<T>

DAL层从数据库中将数据读出来后，传递给BLL层，再通过BLL处理数据后，向WEB层返回，最后到页面上输出，这是一个过程，在这个过程中我需要说两句，ＢＬＬ层与ＷＥＢ层通讯肯定是List,IList等等，即实实在在的数据，而ＤＡＬ层向ＢＬＬ层返回数据时，就需要注意了，因为ＤＡＬ层返回一般是完整的结果集，而在ＢＬＬ层根据具体的业务，再进行字段的过滤，这对于减少数据库与网络传递的压力是有帮助的，下面是一个案值，分别是向ＢＬＬ层以IEnumerable和IQueryable方式进行数据的返回，最后在ＢＬＬ层进行加工数据的操作，看代码：

ＤＡＬ层：

IQueryable方式：

 　　　public IQueryable<WebManageUsers> GetWebManageUsers(ISpecification<WebManageUsers> specification)
        {
            var linq = base.GetEntities(specification).AsQueryable()
                           .Include(i => i.WebManageUser_WebManageRoles)
                           .AsQueryable(); //include会增加链库操作
            return linq;
        }

IEnumerable方式：

  public IEnumerable<WebManageUsers> GetWebManageUsers(ISpecification<WebManageUsers> specification)
        {
            var linq = base.GetEntities(specification).AsQueryable()
                           .Include(i => i.WebManageUser_WebManageRoles)
                           .AsQueryable(); //include会增加链库操作
            return linq;
        }

在ＢＬＬ层进行数据重新组装：

　　　 public List<WebManageUsers> GetWebManageUsers(DateTime? from, DateTime? to, int? status)
        {
            //Create specification
            UserListSpecification dateSpecification = new UserListSpecification(from, to, status);

            return _webManageUsers.GetWebManageUsers(dateSpecification).Select(i => new WebManageUsers_Ext
            {
                CreateDate = i.CreateDate,
                ManageUserID = i.ManageUserID,
                RealName = i.RealName,
            }).ToList<WebManageUsers>();
        }

ＷＥＢ层输出结果，两种方式答到的效果是一样的，但对于一个开发人员来说，只达到效果是远远不够的，因为我们必须要知道ＬＩＮＱ在数据库上究竟干了什么事，即ＬＩＮＱ被翻译成ＳＱＬ语句是什么样的：

IEnumerable方式：

exec sp_executesql N'SELECT 
[Project1].[ManageUserID] AS [ManageUserID], 
[Project1].[LoginName] AS [LoginName], 
[Project1].[Password] AS [Password], 
[Project1].[RealName] AS [RealName], 
[Project1].[Gender] AS [Gender], 
[Project1].[Phone] AS [Phone], 
[Project1].[Mobile] AS [Mobile], 
[Project1].[Email] AS [Email], 
[Project1].[AvatarUrl] AS [AvatarUrl], 
[Project1].[QQ] AS [QQ], 
[Project1].[MSN] AS [MSN], 
[Project1].[Rtx] AS [Rtx], 
[Project1].[Birthday] AS [Birthday], 
[Project1].[Description] AS [Description], 
[Project1].[DepartmentID] AS [DepartmentID], 
[Project1].[CreateDate] AS [CreateDate], 
[Project1].[UpdateDate] AS [UpdateDate], 
[Project1].[Operator] AS [Operator], 
[Project1].[Status] AS [Status], 
[Project1].[IntroductionURL] AS [IntroductionURL], 
[Project1].[WebSystemID] AS [WebSystemID], 
[Project1].[C1] AS [C1], 
[Project1].[ManageUserID1] AS [ManageUserID1], 
[Project1].[ManageRoleID] AS [ManageRoleID], 
[Project1].[CreateDate1] AS [CreateDate1]
FROM ( SELECT 
    [Extent1].[ManageUserID] AS [ManageUserID], 
    [Extent1].[LoginName] AS [LoginName], 
    [Extent1].[Password] AS [Password], 
    [Extent1].[RealName] AS [RealName], 
    [Extent1].[Gender] AS [Gender], 
    [Extent1].[Phone] AS [Phone], 
    [Extent1].[Mobile] AS [Mobile], 
    [Extent1].[Email] AS [Email], 
    [Extent1].[AvatarUrl] AS [AvatarUrl], 
    [Extent1].[QQ] AS [QQ], 
    [Extent1].[MSN] AS [MSN], 
    [Extent1].[Rtx] AS [Rtx], 
    [Extent1].[Birthday] AS [Birthday], 
    [Extent1].[Description] AS [Description], 
    [Extent1].[DepartmentID] AS [DepartmentID], 
    [Extent1].[CreateDate] AS [CreateDate], 
    [Extent1].[UpdateDate] AS [UpdateDate], 
    [Extent1].[Operator] AS [Operator], 
    [Extent1].[Status] AS [Status], 
    [Extent1].[IntroductionURL] AS [IntroductionURL], 
    [Extent1].[WebSystemID] AS [WebSystemID], 
    [Extent2].[ManageUserID] AS [ManageUserID1], 
    [Extent2].[ManageRoleID] AS [ManageRoleID], 
    [Extent2].[CreateDate] AS [CreateDate1], 
    CASE WHEN ([Extent2].[ManageUserID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
    FROM  [dbo].[WebManageUsers] AS [Extent1]
    LEFT OUTER JOIN [dbo].[WebManageUser_WebManageRoles] AS [Extent2] ON [Extent1].[ManageUserID] = [Extent2].[ManageUserID]
    WHERE (@p__linq__0 = 1) AND ([Extent1].[CreateDate] > (CASE WHEN (@p__linq__1 IS NULL) THEN @p__linq__2 ELSE @p__linq__1 END)) AND ([Extent1].[CreateDate] < (CASE WHEN (@p__linq__3 IS NULL) THEN @p__linq__4 ELSE @p__linq__3 END))
)  AS [Project1]
ORDER BY [Project1].[ManageUserID] ASC, [Project1].[C1] ASC',N'@p__linq__0 bit,@p__linq__1 datetime2(7),@p__linq__2 datetime2(7),@p__linq__3 datetime2(7),@p__linq__4 datetime2(7)',@p__linq__0=1,@p__linq__1=NULL,@p__linq__2='0001-01-01 00:00:00',@p__linq__3=NULL,@p__linq__4='9999-12-31 23:59:59.9999999'

IQueryable方式：

exec sp_executesql N'SELECT 
[Extent1].[ManageUserID] AS [ManageUserID], 
[Extent1].[CreateDate] AS [CreateDate], 
[Extent1].[RealName] AS [RealName]
FROM [dbo].[WebManageUsers] AS [Extent1]
WHERE (@p__linq__0 = 1) AND ([Extent1].[CreateDate] > (CASE WHEN (@p__linq__1 IS NULL) THEN @p__linq__2 ELSE @p__linq__1 END)) AND ([Extent1].[CreateDate] < (CASE WHEN (@p__linq__3 IS NULL) THEN @p__linq__4 ELSE @p__linq__3 END))',N'@p__linq__0 bit,@p__linq__1 datetime2(7),@p__linq__2 datetime2(7),@p__linq__3 datetime2(7),@p__linq__4 datetime2(7)',@p__linq__0=1,@p__linq__1=NULL,@p__linq__2='0001-01-01 00:00:00',@p__linq__3=NULL,@p__linq__4='9999-12-31 23:59:59.9999999'

看到这里，您应该有所体会了吧，我们希望的肯定是IQueryable方式的，呵呵，它才是ＬＩＮＱ世界里的延时加载，下面咱们总结一下什么时候用IQueryable方

式来返回数据：

１　结果集很大，需要在ＢＬＬ层去重新组装时

２　使用take skip实现分页时