左外连接给出错误消息'无法强制类型'System.Nullable ' 1'输入'Sys

根据你的评论，问题是你从BoatName, BoatType and BoatOverAllLength尝试create a string，你不能在linq to entities中格式化字符串，正如我之前所说的(之前的帖子，no3)，你可以获取你需要的数据，然后在内存中select创建BoatDets字符串，所以这，肯定是有效的:

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，你可以获取BoatName, BoatType and BoatOverAllLength作为属性，当你的查询获取时，从属性中创建你需要的字符串，像这样:

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
                        };

或者如果你坚持使用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，当您的数据已被提取到内存时，其可读

1-在.netframework4你不能使用enumerations在linq实体，但在.netframework4.5你可以，并在.netframework4当你使用enumeration这个异常发生，虽然我没有看到任何enumeration在你的代码…

2-你可以在你的select属性注释一个一个，找到导致异常的属性，并检查它的类型…

3-可能你不会有任何问题，如果获取数据之前选择他们作为literal像这样:

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-小心，如果jn为空，这行BoatDets = u.jn.BoatName + " [" + u.jn.BoatType + ", " + u.jn.BoatOverAllLength + "]",会导致异常