asp.net错误？？并非所有的代码路径都返回值

private string selClassNum(int Classid)
  {
   string num;
   try
   {
    string con=System.Configuration.ConfigurationSettings.AppSettings["DbPath"];
    //
    OleDbConnection myConn=new OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source="+Server.MapPath(con));
    //
    myConn.Open();
    //
    OleDbCommand cmd=new OleDbCommand("select count(*) as co from Atricle where Classid=?",myConn);
    //
    OleDbParameter clas=cmd.Parameters.Add("@Classid",OleDbType.BigInt,40);
    clas.Value=Classid;
    //
    OleDbDataReader rd;
    //
    rd=cmd.ExecuteReader();
    //
    if(rd.Read())
    {
     num=(string)rd["co"];
     //
     return num;
    }
    rd.Close();
    myConn.Close();    
    //
   }
   catch(Exception e)
   {
    lblMsg.Text=Popmsg.Show(e.Message);
    //
    return num;
   }
  }


谁能给我一个解释？