如何在Linq-to-Entities查询中使用DateTime.AddXXXX函数? [英] How can i use DateTime.AddXXXX functions in a Linq-to-Entities query?

问题描述

我正在尝试在查询中使用AddMonths

List<Entities.Subscriber> items = (from s in context.Subscribers
                 where s.Validated == false && s.ValidationEmailSent == true && s.SubscriptionDateTime < DateTime.Now.AddMonths(-1)
                 select s).ToList();

但是我收到一个错误:

LINQ to Entities无法识别方法'System.DateTime AddMonths(Int32)'方法，并且该方法无法转换为 存储表达式.

我可以在查询中使用此功能吗?

解决方案

最简单的解决方法是使用LINQ在之前算出时间限制:

DateTime limit = DateTime.Now.AddMonths(-1);

List<Entities.Subscriber> items = (from s in context.Subscribers
             where s.Validated == false && s.ValidationEmailSent == true && 
                                           s.SubscriptionDateTime < limit)
             select s).ToList();

或更容易理解的IMO:

var items = context.Subscribers
                   .Where(s => !s.Validated &&
                               s.ValidationEmailSent &&
                               s.SubscriptionDateTime < limit)
                   .ToList();

在此处使用查询表达式没有任何好处，并且与true和false的显式比较是丑陋的IMO(除非您的属性当然是Nullable<bool>类型).

I am trying to use AddMonths in a query

List<Entities.Subscriber> items = (from s in context.Subscribers
                 where s.Validated == false && s.ValidationEmailSent == true && s.SubscriptionDateTime < DateTime.Now.AddMonths(-1)
                 select s).ToList();

But I recieve an error :

LINQ to Entities does not recognize the method 'System.DateTime AddMonths(Int32)' method, and this method cannot be translated into a store expression.

Is there a way I can use this function inside my query?

解决方案

The simplest fix to this is to work out the time limit once before using LINQ:

DateTime limit = DateTime.Now.AddMonths(-1);

List<Entities.Subscriber> items = (from s in context.Subscribers
             where s.Validated == false && s.ValidationEmailSent == true && 
                                           s.SubscriptionDateTime < limit)
             select s).ToList();

Or more readably IMO:

var items = context.Subscribers
                   .Where(s => !s.Validated &&
                               s.ValidationEmailSent &&
                               s.SubscriptionDateTime < limit)
                   .ToList();

There's no benefit in using a query expression here, and explicit comparisons with true and false are ugly IMO (unless your properties are of type Nullable<bool> of course).

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