C# 使用linq查询时出现运行时错误

我能够为Web Api编写此代码。在这段代码中，我编写了两个查询，并以一种基于ctime和startedfollowing的时间合并（而不是合并）注释和跟随者数据的方式组合了这两个查询。如果用户有新的注释，则注释应排在第一位，如果跟随者是第一位的，则跟随者数据应排在第一位

 public IQueryable<Object> GetCommentsandFollowActivityCommnets()
    {

        var combo1 = from c in db.comments
                    join p in db.picturedetails on c.targetpictureid equals p.idpictures
                    join u in db.users on c.iduser equals u.iduser
                    select new TCommentDTO
                    {

                        idcomments=c.idcomments,
                        comment1 = c.comment1,
                        targetpictureid = c.targetpictureid,
                        ctime = c.ctime,
                        iduofpic=p.iduser,
                        iduofcommentor=c.iduser,
                        profilepicofcommentor=u.profilepic,
                        usernameofcommentor=u.username,
                        picFilename=p.picFilename,
                        picTitle=p.picTitle

                    };

      var combo2=  from f in db.followers
                    join u in db.users on f.iduser equals u.iduser
                    select new TfollowerDTO
                    {    
                        idfollowers=f.idfollowers,
                        iduser=f.iduser,
                        targetiduser=f.targetiduser,
                        startedfollowing=f.startedfollowing,
                        unoffollower=u.username,
                        ppoffollower=u.profilepic,
                        status=u.status


                    };
          var result1 = from c in combo1
          select new UserTimeLineDTO
          { SortKey = c.ctime, Member =c};

          var result2 = from c in combo2
          select new UserTimeLineDTO{ SortKey = c.startedfollowing, Member = c };

            var result = result1.Concat(result2).OrderBy(x =>x.SortKey).Select(x => x.Member);

            return result; 

    }

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如何删除此异常？

作为一种解决方法，我将尝试计算内存中的最后一个表达式：

      var result1 = (from c in combo1
      select new UserTimeLineDTO
      { SortKey = c.ctime, Member =c}).ToList();

      var result2 = (from c in combo2
      select new UserTimeLineDTO{ SortKey = c.startedfollowing, Member = c }).ToList();

      var result = result1.Concat(result2).OrderBy(x =>x.SortKey).Select(x => x.Member);

---

联合现在应该成功了，排序和最终投影也应该成功。

。ToList强制计算并在内存中具体化对象。这样，Concat在内存中而不是在数据库端进行计算。

      var result1 = (from c in combo1
      select new UserTimeLineDTO
      { SortKey = c.ctime, Member =c}).ToList();

      var result2 = (from c in combo2
      select new UserTimeLineDTO{ SortKey = c.startedfollowing, Member = c }).ToList();

      var result = result1.Concat(result2).OrderBy(x =>x.SortKey).Select(x => x.Member);