c# List<string>的用法

cs代码：

public List<string> Get_emp_no()
    {
        List<string> emp_no_str = new List<string>();
        string str = "select * from staff_dict where emp_no='" + Session["Emp_No"].ToString() + "' or EMPLOYEE_ID='" + Session["Emp_No"].ToString() + "'";
        OracleDataAdapter da = new OracleDataAdapter(str, StrConn_Oracle);
        DataSet ds = new DataSet();
        da.Fill(ds);
        if (ds.Tables[0].Rows.Count > 0)
        {
            for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
            {
                emp_no_str.Add(ds.Tables[0].Rows[i]["emp_no"].ToString());
            }
            
        }
        return emp_no_str;
    }
    /// <summary>
    /// 初始数据
    /// </summary>
    public void Init_Data()
    {
        string ret_code = "";
        string str_ora = "select * from staff_dict where name ='" + Session["UserName"].ToString() + "'";
        DataSet ds_ora = DbHelperOralce.Query(str_ora);
        // 判断是否 有工号 
        if (ds_ora.Tables[0].Rows.Count > 0)
        {
            string strsql = "";
            // 判断是否是 护士长
            string strqry = "";
            if (Get_emp_no().Count > 0)
            {
                for (int i = 0; i < Get_emp_no().Count; i++)
                {
                    strqry += "leader_emp_no like '%" + Get_emp_no()[i] + "%'"+" or ";
                }
                int l = strqry.Length;
                strqry = strqry.Substring(0, l - 3);
            }
            string str = " select * from Clinical_Dict  where 1=1 and "+strqry+"";
            DataSet ds = DbHelperSQL.Query(str);
            if (ds.Tables[0].Rows.Count > 0)
            {
                int i = ds.Tables[0].Rows.Count;
                for (int j = 0; j < i; j++)
                {
                    ret_code += "'"+ds.Tables[0].Rows[j]["ID"].ToString()+"'" + ",";
                }
                int l = ret_code.Length;
                ret_code = ret_code.Substring(0, l - 1);
                strsql += "   a.dept_code  in (" + ret_code +")";            
            
                string SqlString = "select distinct a.Title_ID, b.title,a.Dept_Code,a.Dept_Name,a.Ygbh,a.YGXM,a.Test_Type,a.test_date,a.Tester,a.Fee_Read_Flag,a.kaohe_times+1 as kaohe_times ,a.kaohe_times as times,a.Feedback,a.apply_range from KaoHe_FeeBack_Detail a left join subject_title b on a.title_id=b.id  where ("+ strsql+") and a.fee_read_flag='0'  and a.submit_flag='1'  and a.Apply_Range=b.Apply_Range order by a.apply_range,a.kaohe_times desc,a.Test_date desc ";
               DataSet ds_sql = DbHelperSQL.Query(SqlString);
               SearchGrird.DataSource = ds_sql.Tables[0];//绑定数据源
               SearchGrird.DataBind();
               Panel1.Visible = true;
            }
           
        }
    }