linq to sql的修改报错解决方案

linq to sql的修改报错
try
  {
  DepotDataContext dataContxt = CommentParameter.DepotContext;
  var vardate = from a in dataContxt.S_MessMessageFeedback
  where a.DJH == InLnqMess.DJH
  select a;
  if (vardate.Count() == 1)
  {
  S_MessMessageFeedback lnqMess = vardate.Single();
   
  lnqMess.SQEvalidateMessage = InLnqMess.SQEvalidateMessage;
  lnqMess.SQEYZRY = BasicInfo.UserName;
  lnqMess.SQEYZRQ = InLnqMess.SQEYZRQ;
  lnqMess.DJZT = "等待质管部确认";
  break;
   
  default:
  break;
  }
   
  dataContxt.SubmitChanges();
  }
  }
  catch (Exception ex)
  {
  m_err = ex.Message;
  return false;
  throw;
  }
然后报错：无法附加已存在的实体！
才开始学习linq to sql，所以请各位帮忙看一下，是不是还有什么没有写？？

[解决办法]
break;

default:
break;

写这些是什么意思？
[解决办法]

C# code

try  {  DepotDataContext dataContxt = CommentParameter.DepotContext;  var vardate = dataContxt.S_MessMessageFeedback.SingleOrDefault(a=>a.DJH == InLnqMess.DJH);    if vardate !=null)  {        vardate .SQEvalidateMessage = InLnqMess.SQEvalidateMessage;  vardate .SQEYZRY = BasicInfo.UserName;  vardate .SQEYZRQ = InLnqMess.SQEYZRQ;  vardate .DJZT = "等待质管部确认";    dataContxt.SubmitChanges();  }         }  catch (Exception ex)  {  m_err = ex.Message;  return false;  throw;  }
[解决办法]
探讨

try
            {
                DepotDataContext dataContxt = CommentParameter.DepotContext;
                var vardate = from a in dataContxt.S_MessMessageFeedback
                            ……

[解决办法]
用 FirstOrDefault