为什么用创建lambda的函数替换IQueryable.Sum选择器中的lambda失败? (错误1025) [英] Why does replacing the lambda in a IQueryable.Sum selector with a function which creates the lambda fail? (Error 1025)

问题描述

我想抽象一个lambda函数，该函数根据状态汇总数据库中的记录.问题是，当我针对LINQ to Entities运行生成的lambda时，出现内部.NET Framework数据提供程序错误1025". (仅供参考，它对LINQ to Objects效果很好.)

I wanted to abstract a lambda function I have which sums up records in my DB based on their status. The problem is, when I run the generated lambda against LINQ to Entities, I get "Internal .NET Framework Data Provider error 1025." (FYI, it works fine against LINQ to Objects).

这是代码:

    <Extension()> _
    Public Function Summarize(ByVal auditsToSummarize As IQueryable(Of HvacAudit)) As IQueryable(Of HvacAuditSummary)
        Return From audit In auditsToSummarize
               Group By month = audit.DateCreated.Value.Month Into g = Group
               Select New HvacAuditSummary With {
                    .GroupingKey = month,
                    .Pending = g.Sum(Function(audit) If(audit.AuditStatusInt = CType(AuditStatus.Pending, Integer), 1, 0)),
                    .Issued = g.Sum(Function(audit) If(audit.AuditStatusInt = CType(AuditStatus.Issued, Integer), 1, 0)),
                    .Closed = g.Sum(Function(audit) If(audit.AuditStatusInt = CType(AuditStatus.Closed, Integer), 1, 0)),
                    .Cancelled = g.Sum(Function(audit) If(audit.AuditStatusInt = CType(AuditStatus.Cancelled, Integer), 1, 0))
                }
    End Function

这是我尝试使用lambda生成器的示例:

And here's an example of how I try to use the lambda generator:

    Select New HvacAuditSummary() With {
        .Cancelled = g.Sum(AuditsWithStatus(AuditStatus.Cancelled))
    }

    Private Function AuditsWithStatus(auditStatus As AuditStatus) As Func(Of HvacAudit, Integer)
        Return Function(audit) If(audit.AuditStatusInt = CType(auditStatus, Integer), 1, 0)
    End Function

注意:我已经看过其他有关此错误的问题，它们似乎都集中在使用错误的LINQ上，因为使用了lambda(Func)代替了表达式.在sum方法中，所有候选人似乎都接受了Func，所以我不知道我还能改变什么.

NOTE: I have looked through other questions with this error, they all seem to focus on using the wrong LINQ because a lambda (Func) was used in place of an expression. In the sum method, all the candidates seem to take a Func, so I don't know what else I can change.

推荐答案

实体框架查询提供程序不知道您的函数AuditsWithStatus. EF尝试将linq语句中的一切转换为sql，并且它只能处理一组有限的.Net函数，而这些函数都知道sql的等效项.其余的linq语句中的所有内容都必须是表达式(Expression<Func<...>>).

The Entity Framework query provider does not know your function AuditsWithStatus. EF tries to translate everything in your linq statement into sql, and it can only handle a restricted set of .Net functions of which it knows the sql equivalent. For the rest everything in the linq statement must be an expression (Expression<Func<...>>).

这意味着:您可以使用函数，但只能在之后中使用ToList()或ToArray()，这会将其转换为linq-to-objects.或者，您必须将函数转换为表达式.

Which means: you can use your function but only after using ToList() or ToArray(), which turns it into linq-to-objects. Or you must turn your function into an expression.

这篇关于为什么用创建lambda的函数替换IQueryable.Sum选择器中的lambda失败? (错误1025)的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！