c# - 为什么 DbFunctions 不能与 Linq to Entities 一起使用

Question

var vals = (from r in result
                    group r by DbFunctions.TruncateTime(r.Created) into g
                    select new { count = g.Count(), date = g});

当我尝试运行上面的代码时，出现以下错误

Results 是数据库查询的结果

"This function can only be invoked from LINQ to Entities."

模型 (result) 返回日期(除其他事项外)，我要做的就是按日期分组(创建)并计算出有多少它存在的时间。

Created 已经是 DateTime

我做错了什么？

Answer 1

DbFunctions 的目的类是将表达式转换为正确的 SQL 语法:

Provides common language runtime (CLR) methods that expose EDM canonical functions for use in DbContext or ObjectContext LINQ to Entities queries.

因为您不是在查询 SQL 源，所以没有理由使用 DbFunctions 方法。

等效的 Linq-to-object 查询是:

var vals = (from r in result
            group r by r.Created.Date into g
            select new { v = g.Count(), d = g});

The issue may be that Created is DateTime?

您可以使用条件运算符来说明这一点:

var vals = (from r in result
            group r by r.Created.HasValue ? (DateTime?)r.Created.Value.Date
                                          : (DateTime?)null 
                into g
            select new { v = g.Count(), d = g});

或者只是

var vals = (from r in result
            group r by r.Created.Value.Date into g
            select new { v = g.Count(), d = g});

如果 Created 不可能为 null(或者如果它是 null 你想抛出异常)