static void Main(string[] args)
        {
            jieguo jg=new jieguo(cff);
            double a, b;
            string s;
            Console.WriteLine("输入a");
            a = Convert.ToDouble(Console.ReadLine());
            Console.WriteLine("输入b");
            b = Convert.ToDouble(Console.ReadLine());
            Console.WriteLine("输入要进行的运算：");
            s=Console.ReadLine();
            switch (s)
            {
                case "*":
                    jg = new jieguo(cff);
                    //s = "8";
                    break;
                case "/":
                    jg = new jieguo(ccf);
                    break;
                default:
                    Console.WriteLine("输入符号有误");
                    //Console.ReadKey();
                    //break;
                    goto end;  《《《《这里不执行break对后面编程有没有影响？
            }
            Console.WriteLine("{0}+{1}={2}",a,b,jg(a,b));
            end:
            Console.ReadKey();