左外部联接给出错误消息“无法将类型'System.Nullable`1'转换为类型'System.Object'。”

Question

这真让我发疯！

我尝试了多种方法（每种方法都进行了多次迭代），但是得到了相同的错误。

我已经在LINQPad中尝试查询，并获得了所需的结果。

场景：一个码头。 如果船被注册为停留在单据上，我想列出所有船单的清单。 如果没有船凭单登记，那么boatID字段可以为NULL（我知道应该将其设置为键，但是我试图使用Linq来获取答案而不更改数据库）。 有一个带有清单清单的“清单”表，其中包括BoatId字段（用于在清单上注册船只的时间）。 第二个表是“ Boat”表，以BoatId为键，并提供其他船详细信息。

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;

这是产生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为类型System.Object 。 LINQ to Entities仅支持强制转换EDM基本类型或枚举类型。

我已经研究了在本网站（以及其他网站）上可以找到的尽可能多的解决方案，但是我似乎做对了。

Answer 1

1-在.netframework4中，您不能在linq中使用实体enumerations ，但是在.netframework4.5中，您可以使用enumeration ；在.netframework4中，当您使用enumeration时，会发生此异常，尽管我在您的代码中看不到任何enumeration ...

2-您可以在select属性中逐一注释属性，以查找导致异常的属性，并检查其类型...

3-如果在选择数据作为literal之前先获取数据，则可能不会有任何问题，如下所示：

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-请注意，如果jn为null，则这行BoatDets = u.jn.BoatName + " [" + u.jn.BoatType + ", " + u.jn.BoatOverAllLength + "]",引起异常

Answer 2

根据您的评论，问题是您尝试从BoatName, BoatType and BoatOverAllLength create a string ，您无法像我之前所说的那样在linq to entities BoatName, BoatType and BoatOverAllLength字符串格式化linq to entities （上一篇文章，no3），您可以获取所需的数据并然后在内存中select以创建BoatDets字符串，因此这肯定可以工作：

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，您可以获取BoatName, BoatType and BoatOverAllLength作为属性，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：在我的最后两个查询中， BoatDets属性不能在linq to entity BoatDets ，并且在将数据提取到内存时可读