LINQ將DateTime轉換爲字符串

Question

List<Post> list = 
(
    from c in db.TitleComments 
    join t in db.Titles on c.TitleId equals t.Id 
    join u in db.Users on c.UserId equals u.Id 
    where t.Id == _titleId && c.Date > time 
    orderby c.Date descending 
    select new Post { Username = u.Username, PostingDate = c.Date.ToString(), Data = c.Comment } 
).ToList(); 

上面的代碼導致日期轉換爲字符串PostingDate = c.Date.ToString（）時發生異常。任何想法如何解決這個問題？

異常錯誤： {「LINQ到實體無法識別方法‘System.String的ToString（）’方法，而這種方法不能被翻譯成店表達」}

Answer 1

LINQ試圖使用SQL日期轉換爲字符串，但因爲是在SQL沒有ToString（）方法就不能它轉換，這種現象是由設計 - 喬金 -

換句話說，返回日期本身和它在執行SQL上偏後轉換爲字符串：

(
select new { Username = u.Username, 
    PostingDate = c.Date 
    [...] 
}) 
.ToList() // runs on SQL and returns to the application 
.Select(o => // is not generating a SQL, it is running on the app 
    new Post { Username = o.Username, 
     PostingDate = o.PostingDate.ToString(), 
     [...] 
    }) 

Answer 2

您可以補救的問題通過投影爲匿名類型，然後在數據已經從數據庫返回後，在稍後的步驟中輸入到Post。

(from .... 
select new { /* stuff */, Date = c.Date }) 
.AsEnumerable() 
.Select(p => new Post { /* stuff */, PostingDate = p.Date.ToString() }) 
.ToList(); 

然而，鑑於你有一個名爲PostingDate財產的原始來源該日期，我建議你修改你的對象實際上保持值作爲DateTime而不是字符串。

Answer 3

我不認爲這可以直接完成。

var list =  
    select new Post { Username = u.Username, PostingDate = SqlFunctions.StringConvert(c.Date), Data = c.Comment } 
from 
(from c in db.TitleComments 
    join t in db.Titles on c.TitleId equals t.Id 
    join u in db.Users on c.UserId equals u.Id 
    where t.Id == _titleId && c.Date > time 
    orderby c.Date descending).AsEnumerable() 
    ).ToList(); 

還與EF4你可以嘗試這樣的事：

List<Post> list = 
(
from c in db.TitleComments 
join t in db.Titles on c.TitleId equals t.Id 
join u in db.Users on c.UserId equals u.Id 
where t.Id == _titleId && c.Date > time 
orderby c.Date descending 
select new Post { Username = u.Username, PostingDate = SqlFunctions.DateName(c.Date), Data = c.Comment } 
).ToList();