DataTable 手动排序 无效的问题解决方法？？

DataTable dt=new DataTable();
   DataColumn dc;
   DataRow dr;
            dc=new DataColumn("ID",typeof(System.Int32));
   dt.Columns.Add(dc);
   dc=new DataColumn("SUBJECT",typeof(System.String));//标题
   dt.Columns.Add(dc);
   dc=new DataColumn("DISPLAY_NAME",typeof(System.String));//提交部门
   dt.Columns.Add(dc);
   dc=new DataColumn("CO_DEPT_NAME",typeof(System.String));//受理部门
   dt.Columns.Add(dc);
   dc=new DataColumn("ISSUE_DATE",typeof(System.DateTime));//提交时间
   dt.Columns.Add(dc);
            
   dc=new DataColumn("DEADLINE",typeof(System.String));//回复情况
   dt.Columns.Add(dc);
 
   dc=new DataColumn("Num",typeof(System.Int32));//办理情况 排序 //红灯=3 黄灯=2 绿灯=1
   dt.Columns.Add(dc);

   dc=new DataColumn("CO_TYPE",typeof(System.String));
   dt.Columns.Add(dc);

   dc=new DataColumn("FEEDBACK_FLAG",typeof(System.String));
   dt.Columns.Add(dc);

   string n=System.Configuration.ConfigurationSettings.AppSettings["ZnNum"].ToString();
   int num=int.Parse(n);
   string sql="SELECT  top "+num+"    CO_TYPE,DEPT_INFO.DISPLAY_NAME, CO_BASEINFO.SUBJECT, CO_BASEINFO.CO_DEPT_NAME,";
   sql+=" CO_BASEINFO.ISSUE_DATE, CO_BASEINFO.PUBLISH_DATE, CO_BASEINFO.ID,CO_BASEINFO.FEEDBACK_FLAG,CO_BASEINFO.DEADLINE";
   sql+=" FROM         CO_BASEINFO INNER JOIN";
   sql+=" DEPT_INFO ON CO_BASEINFO.ISSUE_DEPT_ID = DEPT_INFO.ID where CO_BASEINFO.ISSUE_FLAG='1' and CO_BASEINFO.CO_TYPE not in('police_abroad','customs_abroad') and CO_BASEINFO.OPEN_FLAG=1 order by FEEDBACK_FLAG asc,ISSUE_DATE desc";
   
   DataTable dt2=GetNewTableByHb2004(sql);
   for(int i=0;i<dt2.Rows.Count;i++)
   {
      dr=dt.NewRow();
      dr["ID"]=Convert.ToInt32(dt2.Rows[i]["ID"].ToString());
      dr["SUBJECT"]=dt2.Rows[i]["SUBJECT"].ToString();
      dr["DISPLAY_NAME"]=dt2.Rows[i]["DISPLAY_NAME"].ToString();
      dr["CO_DEPT_NAME"]=dt2.Rows[i]["CO_DEPT_NAME"].ToString();
      dr["ISSUE_DATE"]=Convert.ToDateTime(dt2.Rows[i]["ISSUE_DATE"].ToString());
      dr["DEADLINE"]=dt2.Rows[i]["DEADLINE"].ToString();
      dr["CO_TYPE"]=dt2.Rows[i]["CO_TYPE"].ToString();
      dr["FEEDBACK_FLAG"]=dt2.Rows[i]["FEEDBACK_FLAG"].ToString();
    //判断处理情况
    int ifnum=IsClose2(dt2.Rows[i]["ISSUE_DATE"].ToString(),dt2.Rows[i]["DEADLINE"].ToString(),3,dt2.Rows[i]["FEEDBACK_FLAG"].ToString());
    dr["Num"]=ifnum;
    dt.Rows.Add(dr);
     
   }
   //**************下面才能实现按照需要的排序输出******************//
   DataRow[] rows=dt.Select("","Num desc,ISSUE_DATE desc");
   DataTable tmptb=dt.Clone();
   tmptb.Rows.Clear();
   foreach(DataRow row in rows)
    tmptb.ImportRow(row);

//**************************************************//
   return tmptb;