随机生成+－*/运算的简单算法

for(int i = 0; i<99 ;i++)
        {
        Random rnd = new Random();
        //运算符
        int opera = rnd.Next(1,5);
        //opera = 4;
        int result = 0;
        int num1 = 0;
        int num2 = 0;
        int input;
        do
        {
            result = rnd.Next(20);
        }while(result < 1);
        char op = '\0';
        switch(opera)
        {
            case 1: //加
                
            //Console.WriteLine("{0}",result);
                num1 = rnd.Next(1,result);
                //Console.WriteLine(num1);
                 num2 = result - num1;
                //Console.WriteLine(num2);
                op = '+';
                
                    break;
            case 2: //减
                num2 = rnd.Next(1,result);
                num1 = result+num2;
                op = '-';
                //Console.WriteLine("{0}-{1}=?",num1,num2);
                break;
            case 3://乘
                if(result%2!=0)result++;
                num1 = rnd.Next(1,result);
                //if(result%num1!=0)num1++;
                num2 = result / num1;
                //if(result%num1!=0)num2++;
                result = (result==(num1 * num2)?result:(num1*num2));
                op = '*';
                break;
            case 4://除
                result = rnd.Next(1,10);
                if(result%2!=0)result++;
                num2 = rnd.Next(1,result);
                num1 = result * num2;
                op = '/';
                break;
                
        }
        
        Console.WriteLine("-------\n{0}{2}{1}=?({3})",num1,num2,op,result);
                
                int.TryParse(Console.ReadLine() as string,out input);
                Console.WriteLine(input==result);
            
        }