用C#写的四则运算类(包括括号运算)

using System;
using System.Collections;
using System.Text;

namespace ExpresstionClass
{
    public class ExpresstionClass
    {
        public string Expresstion;//String类型表达式

        public ExpresstionClass()
        {
       
        }

        public ExpresstionClass(string expresstion)
        {
            this.Expresstion = expresstion;
        }

        // 判断字符串是否为数值
        public bool IsNumberExp(string str)
        {
            bool isnumeric = false;
            byte c;
            if (str == null || str.Length == 0)
                return false;
            System.Text.ASCIIEncoding ascii = new System.Text.ASCIIEncoding();
            byte[] bytestr = ascii.GetBytes(str);
            for (int i = 0; i < bytestr.Length; i++)
            {
                c = bytestr[i];
                if ((c >= 48 && c <= 57) || c == 46)
                {
                    isnumeric = true; ;
                }
                else
                {
                    if (c == 45 && bytestr .Length  > 1)
                    {
                        isnumeric = true;
                    }
                    else
                    {
                        isnumeric = false;
                        break;
                    }
                }
            }
            return isnumeric;
        }

        // 基本一目计算
        public double account(double n1, double n2, string num_op)
        {
            double aresult = 0;
            switch (num_op)
            {
                case "+":
                    aresult = n1 + n2;