asp.net-mvc LINQ 将日期时间转换为字符串
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LINQ convert DateTime to string
提问by Ronald
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();
The code above causes exception on the convertion of date to string, PostingDate = c.Date.ToString(). Any ideas how to get around this?
上面的代码在将日期转换为字符串时会导致异常，PostingDate = c.Date.ToString()。任何想法如何解决这个问题？
Exception error: {"LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression."}
异常错误：{“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。”}
回答by BrunoLM
linq is trying to convert date to string using sql but since there is no ToString() method in sql it can't convert it, this behavior is by design- Joakim
linq 正在尝试使用 sql 将日期转换为字符串，但由于 sql 中没有 ToString() 方法，因此无法将其转换，此行为是设计使然 - Joakim
In other words, return the date itself and convert it to a string after it executes on SQL side:
换句话说，返回日期本身并在它在 SQL 端执行后将其转换为字符串：
(
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(),
[...]
})
回答by Anthony Pegram
You can remedy your problem by projecting into an anonymous type, and then at a later step project into Postafter the data has already been returned from the DB.
您可以通过投影到匿名类型来解决您的问题，然后在Post数据已经从数据库返回后的稍后步骤投影到。
(from ....
select new { /* stuff */, Date = c.Date })
.AsEnumerable()
.Select(p => new Post { /* stuff */, PostingDate = p.Date.ToString() })
.ToList();
However, given that you have a property called PostingDate, the original source being a date, I would recommend your revise your object to actually keep the value as a DateTimeinstead of a string.
但是，鉴于您有一个名为PostingDate的属性，原始来源是 date，我建议您修改对象以实际将值保留为 aDateTime而不是字符串。
回答by Baz1nga
I dont think this can be done in a direct way.
我认为这不能以直接的方式完成。
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();
Also with EF4 you can try something like this:
同样使用 EF4，您可以尝试这样的操作：
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();
