（常用）C#获取sql查询所返回的字段值

qlConnection conn = new SqlConnection(conn_str);
            conn.Open();
            string sql = "select used_number from pipeline_Number";
            SqlCommand cmd = new SqlCommand(sql, conn);
            SqlDataReader rdr = cmd.ExecuteReader();
            while (rdr.Read())
            {
                Console.WriteLine(rdr[0]);
            }
            Console.WriteLine("bye bye");
            Console.ReadKey();