混合运算（控制台）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;

namespace ConsoleApplication2
{
    class Program
    {
        static int sun;
        private static int x, y, z;
        private static string  f;
        private static string  h;
        static void Main(string[] args)
        {
            Console.Write("请输入随机数的范围(9-10)：");
            string azzz = Console.ReadLine();
            string regex = @"(\d+)\D+(\d+)";
            Match mstr = Regex.Match(azzz, regex);
            int s1 = int.Parse(mstr.Groups[1].Value);
            int s2 = int.Parse(mstr.Groups[2].Value);
            random(s1,s2);
            Console.Write("请输入答案：");
            int l = int.Parse(Console.ReadLine());
            rightanswer(f, h, x, y, z);
            if (l == sun)
            {
                Console.WriteLine("回答正确！");
            }
            else
            {
                Console.WriteLine("回答错误！");
            }
            Console.Read();
        }
        private static void random(int a,int b)
        {
            Random si = new Random();
            x = si.Next(a, b);
            y = si.Next(a, x);
            z = si.Next(a, y);
            string[] operators = new string[] { "+", "-", "*", "/" };
             f = operators[new Random().Next(0, 4)];
             h = operators[new Random().Next(0, 4)];
            string z1 = "=";
            string xfyhz = x + f + y + h + z + z1;
            Console.WriteLine(xfyhz);

        }
        private static int rightanswer(string a1, string a2, int c, int d, int e)
        {
            string a12 = a1 + a2;
            switch (a12)
            {
                case "++":
                    sun = c + d + e;
                    break;
                case "+-":
                    sun = c + d - e;
                    break;
                case "-+":
                    sun = c - d + e;
                    break;
                case "--":
                    sun = c - d - e;
                    break;
                case "-*":
                    sun = c - d * e;
                    break;
                case "+*":
                    sun = c + d + e;
                    break;
                case "*/":
                    sun = c * d / e;
                    break;
                case "**":
                    sun = c * d * e;
                    break;
                case "*-":
                    sun = c + d - e;
                    break;
                case "*+":
                    sun = c * d + e;
                    break;
                case "/*":
                    sun = c / d * e;
                    break;
                case "/-":
                    sun = c / d - e;
                    break;
                case "/+":
                    sun = c / d + e;
                    break;
                case "//":
                    sun = c / d / e;
                    break;
                case "-/":
                    sun = c - d / e;
                    break;
                case "+/":
                    sun = c + d / e;
                    break;
            }

            return sun;

        }
    }
}

总结：
改了很长时间，真是一点细微的细节都能决定程序的运行是否成功！写代码应该仔细，仔细，再仔细！