public class GradeService
    {
        public List<Grade> GetAllGrade()
        {
            List<Grade>  gradeList =new List<Grade>();
            SqlConnection sqlConn = null;
            SqlCommand sqlComm = null;
            try
            {
                string sql = string.Format("SELECT * FROM Grade");
                sqlConn = DBHelper.GetSqlConn();
                sqlComm = new SqlCommand(sql,sqlConn);
                SqlDataReader sqlDR = sqlComm.ExecuteReader();
                while(sqlDR.Read())
                {
                    Grade grade = new Grade();
                    grade.GradeId =Convert.ToInt32(sqlDR["GradeID"].ToString());
                    grade.GradeName = sqlDR["GradeName"].ToString();

                    gradeList.Add(grade);

                }
                return gradeList;
            }
            catch (Exception ex)
            {
                throw ex;
            }
           finally
            {
                sqlConn.Close();
            }
        }
    }

明白了   谢啦