Linq to Sql 左连接 , 取右表可能为 null的 int类型字段

                var queryLeft = from c in db.AA
                                join sr in
                                    (
                                     from ss in db.BB
                                     select ss
                                     )
                                   on c.Id equals sr.Sid
                                   into x
                                from y in x.DefaultIfEmpty()
                                select new { c.Id, c.timeflag, Tzsbh = y.Tzsbh };

                 result = from c in queryLeft.AsEnumerable()
                             select new ShowInfo
                             {
                                 Sid = c.Id,
                                
                                 timeflag = c.timeflag,
                                 PdfId = c.Tzsbh.ToString()  //int 转 string 这一行, 如果对象为null,报错
                             };

 var sg = db.AA.GroupJoin(db.BB, c => c.Id, d => d.Sid, (c, d) => new { c.Id, c.timeflag, Pdf = d.FirstOrDefault() }).Select(o => o);

 var result = from c in sg.AsEnumerable()
                             select new ShowInfo
                             {
                                 Sid = c.Id,
                                 timeflag = c.timeflag,
                                 PdfId = c.Pdf == null ? "" : c.Pdf.Tzsbh.ToString()
                             };