为什么DropDownList受保护级别限制

E:\ps\zyp\control\post.ascx.cs(48): 不可访问“System.Web.UI.WebControls.ListControl.items”，因为它受保护级别限制
private void GetSourceListData()
        {//清空DropDownList控件的所有数据项
            SoureList.items.Clear();
            //创建数据库的连接对象
            String cmdText="SECECT powerid,powername from power";
            SqlConnection myConnection=new SqlConnection(SQLCONNECTIONSTRING);
            SqlCommand myCommand=new SqlCommand(cmdText,myConnection);
            myConnection.Open();
            SqlDataReader reader=myCommand.ExecuteReader();
            while(reader.Read())
            {
                SoureList.items.Add(new ListItem(reader["prowerid"].ToString(),reader["powername"].ToString()));
            }
            reader.Close();
            myConnection.Close();

        }