這段代碼，運行到WHILE時，會提示閱讀器關閉，無法read。為什么閱讀器會關閉呢？求解釋
public   List<StudentClass> GetAllClasses()
  {
  String Sql = "select ClassID,ClassName from StudentClass ";
  SqlDataReader objDataReader = SQLHelper.GetReader(Sql);
    
  List<StudentClass> list = new List<StudentClass>();
       while (objDataReader.Read())--------就是這一步
      {
          list.Add(new StudentClass
          {
              ClassID = Convert.ToInt32(objDataReader["ClassID"]),
              ClassName = Convert.ToString(objDataReader["ClassName"])
          });
        
          


      }
      objDataReader.Close();
      return list;

  }

objDataReader依賴于數據庫連接，大概率是數據庫連接關閉了