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？？

------解决方案--------------------
break;

default:
break;

写这些是什么意思？
------解决方案--------------------

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;
  }

------解决方案--------------------
探讨

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


------解决方案--------------------
用 FirstOrDefault