在LINQ查询中使用ToString()? [英] Using ToString() in LINQ queries?

问题描述

我已经写了一个LINQ查询来填充列表视图，但是它使用了.ToString()方法，这显然是不允许的.当我使用下面的代码时，我收到错误消息:

I have writting a LINQ query to fill a listview but it useses the .ToString() method which apparetly is not allowed. When I use the below code I get the error message:

错误:LINQ to Entities无法识别方法'System.String ToString()'，并且该方法无法转换为商店表达式

Error: LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression

有没有一种方法可以在LINQ中使用ToString()，或者如果不可能，在查询中将DateTime转换为String的解决方案是什么?请注意，ReleaseDateName是字符串，ReleaseDate是DateTime

Is there a way to use the ToString() in LINQ or if that is not possible what is the solution to converting a DateTime to String in the query. Please knot that ReleaseDateName is a string and ReleaseDate is a DateTime

using (var db = new ReleaseInfo())
{
    lvReleaseInfo.DataSource = (from r in db.MediaReleases
                                join rn in db.ReleaseNames
                                on new { MediaReleaseID = r.MediaReleaseID, CultureCodeID } equals new { rn.MediaReleaseID, rn.CultureCodeID }
                                join plat in db.MediaPlatforms
                                on new { MediaPlatformID = r.MediaPlatformID, CultureCodeID } equals new { plat.MediaPlatformID, plat.CultureCodeID }
                                join pub in db.MediaPublishers
                                on new { MediaPublisherID = r.MediaPublisherID, CultureCodeID } equals new { pub.MediaPublisherID, pub.CultureCodeID }
                                join c in db.Countries
                                on new { CountryID = r.CountryID, CultureCodeID } equals new { c.CountryID, c.CultureCodeID }
                                join rd in db.ReleaseDates
                                on new { MediaReleaseID = r.MediaReleaseID, CultureCodeID } equals new { rd.MediaReleaseID, rd.CultureCodeID }
                                join a in db.AffiliateLinks
                                on new { MediaReleaseID = r.MediaReleaseID, CultureCodeID } equals new { a.MediaReleaseID, a.CultureCodeID }
                                where r.SectionID == SectionID
                                select new
                                {
                                    rn.ReleaseTitle,
                                    plat.MediaPlatformName,
                                    pub.MediaPublisherName,
                                    c.CountryName,
                                    ReleaseDate = (rd.ReleaseDate == null ? rd.ReleaseDateName : rd.ReleaseDate.ToString()),
                                    a.AffiliateLinkAddress
                                }).ToList();
    lvReleaseInfo.DataBind();
}

推荐答案

由于您正在具体化要列出的查询，因此可以在.NET端而不是在RDBMS中进行转换，如下所示:

Since you are materializing your query to list anyway, you could do the conversion on the .NET side, rather than in the RDBMS, like this:

...
select new {
   rn.ReleaseTitle,
   plat.MediaPlatformName,
   pub.MediaPublisherName,
   c.CountryName,
   rd.ReleaseDateName,
   rd.ReleaseDate,
   a.AffiliateLinkAddress
}).AsEnumerable() // <<== This forces the following Select to operate in memory
.Select(t => new {
   t.ReleaseTitle,
   t.MediaPlatformName,
   t.MediaPublisherName,
   t.CountryName,
   ReleaseDate = t.ReleaseDateName ?? t.ReleaseDate.ToString()
   t.AffiliateLinkAddress        
}).ToList();

由于在IEnumerable<T>中的元素上调用了ToString()，因此它将不再失败.还要注意，使用??运算符代替了对空检查? :的条件.

Since the ToString() is called on an element from IEnumerable<T>, it will no longer fail. Also note the use of ?? operator in place of a null-checking ? : conditional.

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