C# MySql Select

MySqlCommand objCmd = new MySqlCommand("select * from `main_db`.`t_realdailyinfo` ", objConn);
            MySqlCommand objCmd = new MySqlCommand("select * from `main_db`.`t_user` where SysNo = 1 ", objConn);
            MySqlCommand objCmd = new MySqlCommand("select UserName from `main_db`.`t_user` where SysNo = 2 ", objConn);

// UserName ,SysNo 均为column名称

MySqlDataReader  r = objCmd.ExecuteReader();
            while (r.Read())
            {             
       
                    //FieldCount为这个表中有多少的列 column
                    int i = r.FieldCount;
                    textBox3.Text = i.ToString();
            }