这让我疯了！
我尝试了很多种方法（以及每种方式的多次迭代），但得到了同样的错误。
我在LINQPad中尝试过查询并获得所需的结果。
场景：码头。我想要一张带有船只细节的所有单据的清单，如果一艘船被注册为停留在滑道上。如果在滑动处没有注册船只，则boatID字段可能为NULL（我知道这应该设置为密钥，但我尝试使用Linq来获得答案而不更改数据库）。有一个带有滑动清单的“滑动”表，包括一个BoatId区域（当船只在滑道上注册时）。第二张桌子是一张'Boat'牌桌，以BoatId作为钥匙和其他船只细节。
这是一个SQL查询（产生我想要的结果）：
Select s.SlipID, s.SlipNumber, s.Length, s.Electricity, s.Telephone, s.TV,
b.BoatName+' ['+b.BoatType+', '+convert(varchar,b.BoatOverAllLength)+']' as boatDets,
s.Status
from Slip as s left outer join boat as b on s.BoatID = b.BoatId;
以下是给出错误的解决方案之一（但在LINQPad中有效）：
var slipDets6 = from s6 in db.Slips
join b6 in db.Boats on s6.BoatId equals b6.BoatId into temp
from jn in temp.DefaultIfEmpty()
orderby s6.SlipNumber
select new
{
SlipID = (int?) s6.SlipId,
SlipNumber = s6.SlipNumber,
Length = s6.Length,
Electricity = s6.Electricity,
Telephone = s6.Telephone,
TV = s6.TV,
BoatDets = jn.BoatName + " [" + jn.BoatType + ", " + jn.BoatOverAllLength + "]",
Status = s6.Status
};
我收到的实际错误代码是：
无法将类型System.Nullable'1强制转换为System.Object类型。 LINQ to Entities仅支持转换EDM原语或枚举类型。
我已经深入研究了我可以在这个网站（和其他网站）上找到的解决方案，但我似乎正在做正确的事情。
答案 0 :(得分：0)
1- in .netframework4你不能在linq中使用enumerations到实体，但在.netframework4.5中你可以，在.netframework4中使用enumeration这个异常发生时，虽然我看不到您的代码中的任何enumeration ...
2-您可以逐个评论select中的属性，找到导致异常的属性，并检查其类型......
3-可能你没有任何问题，如果获取数据之前选择它们为literal这样：
var slipDets6 = (from s6 in db.Slips
join b6 in db.Boats on s6.BoatId equals b6.BoatId into temp
from jn in temp.DefaultIfEmpty()
orderby s6.SlipNumber
select new {s6, jn})
.ToList()
.Select(u => new
{
SlipID = (int?)u.s6.SlipId,
SlipNumber = u.s6.SlipNumber,
Length = u.s6.Length,
Electricity = u.s6.Electricity,
Telephone = u.s6.Telephone,
TV = u.s6.TV,
BoatDets = u.jn.BoatName + " [" + u.jn.BoatType + ", " + u.jn.BoatOverAllLength + "]",
Status = u.s6.Status
})
.ToList();
4-注意，如果jn为空，则此行BoatDets = u.jn.BoatName + " [" + u.jn.BoatType + ", " + u.jn.BoatOverAllLength + "]",会导致异常
答案 1 :(得分：0)
根据您的评论，问题是您尝试从create a string BoatName, BoatType and BoatOverAllLength，你不能格式化linq to entities中的字符串，正如我之前所说（prev post，no3），你可以获取你需要的数据，然后在内存中select来创建BoatDets字符串，所以这肯定有效：
var slipDets6 = (from s6 in db.Slips
join b6 in db.Boats on s6.BoatId equals b6.BoatId into temp
from jn in temp.DefaultIfEmpty()
orderby s6.SlipNumber
select new {s6, jn})
.ToList()
.Select(u => new
{
SlipID = (int?)u.s6.SlipId,
SlipNumber = u.s6.SlipNumber,
Length = u.s6.Length,
Electricity = u.s6.Electricity,
Telephone = u.s6.Telephone,
TV = u.s6.TV,
BoatDets = u.jn == null ? "" : u.jn.BoatName + " [" + u.jn.BoatType + ", " + u.jn.BoatOverAllLength + "]",
Status = u.s6.Status
})
.ToList();
或者，您可以将BoatName, BoatType and BoatOverAllLength作为属性获取，并在获取查询时，从该属性创建所需的字符串，如下所示：
public class FlatSlip
{
public int? SlipID { get; set; }
public string SlipNumber { get; set; }
public string Length { get; set; }
public string Electricity { get; set; }
public string Telephone { get; set; }
public string TV { get; set; }
public string BoatName { get; set; }
public string BoatType { get; set; }
public string BoatOverAllLength { get; set; }
public string Status { get; set; }
public string BoatDets
{
get
{
return this.BoatName + " [" + this.BoatType + ", " + this.BoatOverAllLength + "]";
}
}
}
var slipDets6 = from s6 in db.Slips
join b6 in db.Boats on s6.BoatId equals b6.BoatId into temp
from jn in temp.DefaultIfEmpty()
orderby s6.SlipNumber
select new FlatSlip()
{
SlipID = (int?)s6.SlipId,
SlipNumber = s6.SlipNumber,
Length = s6.Length,
Electricity = s6.Electricity,
Telephone = s6.Telephone,
TV = s6.TV,
BoatName = jn == null ? "" : jn.BoatName,
BoatType = jn == null ? "" : jn.BoatType,
BoatOverAllLength = jn == null ? "" : jn.BoatOverAllLength,
Status = s6.Status
};
或者如果您坚持使用literal：
public class Boat
{
public Boat()
{
}
public Boat(string BoatName, string BoatType, string BoatOverAllLength)
{
this.BoatName = BoatName;
this.BoatType = BoatType;
this.BoatOverAllLength = BoatOverAllLength;
}
public string BoatName { get; set; }
public string BoatType { get; set; }
public string BoatOverAllLength { get; set; }
public string BoatDets
{
get
{
return this.BoatName + " [" + this.BoatType + ", " + this.BoatOverAllLength + "]";
}
}
}
var slipDets6 = (from s6 in db.Slips
join b6 in db.Boats on s6.BoatId equals b6.BoatId into temp
from jn in temp.DefaultIfEmpty()
orderby s6.SlipNumber
select new { s6, jn })
.ToList()
.Select(u => new
{
SlipID = (int?)u.s6.SlipId,
SlipNumber = u.s6.SlipNumber,
Length = u.s6.Length,
Electricity = u.s6.Electricity,
Telephone = u.s6.Telephone,
TV = u.s6.TV,
BoatDets = jn == null ? new Boat() : new Boat(u.jn.BoatName, u.jn.BoatType, u.jn.BoatOverAllLength),
Status = u.s6.Status
})
.ToList();
注意：BoatDets属性，在我的最后两个查询中，不能在linq to entity中使用，并且当您的数据被提取到内存时它是可读的