递归实现表达式求值 C#版本

思想和网上很多人一样，
就是把表达式字符串expression看作是一个个item组成的值，
item之间是最简单的+ - 关系，
item由一个个factor组成的值，item之间的关系是*/关系，
factor是数字或表达式等基本单位

using System;
namespace 表达式求值算法
{
    class Program
    {
        public static void Main(string[] args)
        {
            int current = 0;
            Console.WriteLine(new Expression("(2+3)*(5+7)+9/3").calculate().ToString("0.00"));
        }
    }
    //先递归实现+-*/等操作
    class Expression{
        private string exp;
        private bool isNum(char ch)
        {
            return (ch >= '0' && ch <= '9')||(ch=='.');
        }
        public Expression(string str)
        {
            exp = str;
        }
        //  +  -
        public double calculate()
        {
            return expression_value( ref current);
        }
        private int current = 0;
        private double expression_value(ref int current)
        {
            double result = item_value(ref current);
            bool more = true;
            while((current < exp.Length)&& more)
            {
                char op = exp[current];
                if (op == '+' || op == '-')
                {
                    current += 1;
                    double numTemp = item_value(ref current);
                    if (op == '+')
                    {
                        result += numTemp;
                    }
                    else if (op == '-')
                    {
                        result -= numTemp;
                    }
                }
                else
                    more = false;
            }
            return result;
        }
        // *  /
        private double item_value(ref int current)
        {
            double result = factor_value(ref  current);
            bool more = true;
            while ((current<exp.Length)&& more)
            {
                char op = exp[current];
                if ( op =='*'||op=='/')
                {
                    current += 1;
                    double numTemp = factor_value(ref current);
                    if (op == '*')
                    {
                        result *= numTemp;
                    }
                    else if (op== '/')
                    {
                        result /= numTemp;
                    }
                } 
                else
                    more = false;
            }
            return result;
        }
        //判断是不是(，再一次调用exprssion_value()
        private double factor_value(ref int current)
        {
            double result = 0;
            if (current < exp.Length)
            {
                char opt = exp[current];                
                if (opt == '(')
                {
                    current += 1;
                    result = expression_value(ref current);
                    current += 1;//去掉)符号
                }
                else
                {
                    string numTemp = opt.ToString();
                    current += 1;
                    while ( (current <exp.Length) && isNum(exp[current]))//后面还有数
                    {
                        numTemp += exp[current];
                        current += 1;
                    }                   
                    result = double.Parse(numTemp);
                }
            }
            return result;
        }
    }
}