4-7-

double dbA;
            string strA;

            Console.WriteLine("   学生成绩管理系统");
            Console.WriteLine("   输入（-1退出系统）");


            do
            {
                Console.Write("请输入学生的成绩：");
                dbA = Convert.ToDouble(Console.ReadLine());
                if (dbA == -1)
                {
                    break;
                }


                if (dbA < 0 || dbA > 100)
                {
                    Console.WriteLine("您输入的成绩不合法请重新输入");
                    continue;
                }

                switch ((int)(dbA / 10))
                {
                    case 10:
                    case 9:
                        strA = "优秀";
                        break;
                    case 8:
                        strA = "良";
                        break;
                    case 7:
                        strA = "中";
                        break;
                    case 6:
                        strA = "及格";
                        break;
                    default:
                        strA = "不及格";
                        break;

                }

                Console.WriteLine(strA);
                strA = "";
                dbA = 0;
            } while (true);
            Console.WriteLine("感谢使用！");
            Console.ReadLine();