在LINQ查询中使用ToString()？

Question

我编写了一个LINQ查询来填充listview,但它使用的是.ToString()方法,这种方法显然是不允许的.当我使用下面的代码时,我收到错误消息:

错误:LINQ to Entities无法识别方法'System.String ToString()'方法,并且此方法无法转换为存储表达式

有没有办法在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.请知道ReleaseDateName是一个字符串,ReleaseDate是一个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();
}

Answer 1

既然您要将列表的查询具体化,那么您可以在.NET端进行转换,而不是在RDBMS中进行转换,如下所示:

...
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ToString()从一个元素调用IEnumerable<T>,它将不再失败.还要注意使用??运算符代替空值检查? :条件.