string connectionStr = "Server = DESKTOP-IDK01B1\\SQLEXPRESS2014; DataBase = StudentManageDB; Uid = sa; Pwd = sa";
            SqlConnection conn = new SqlConnection(connectionStr);
            conn.Open();
            string sql = "select * from Students";
            SqlCommand command = new SqlCommand(sql, conn);
            //执行查询方法，返回DataReader对象
            SqlDataReader objReader = command.ExecuteReader();
            //读取数据
            while(objReader.Read())//用Read()方法判断是否有数据
            {
                Console.WriteLine(objReader["StudentId"].ToString() + "\t" + objReader["StudentName"] + "\t" + objReader["Gender"]);
            }
            //关闭读取器
            objReader.Close();
            conn.Close();//先关闭读取器，再关闭连接，顺序不可调换