Calculator using single readline c#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CalculatorApplication
{
    class Calculator
    {
        string input;
        char[] szArr;
        int length ;
        int current = -1;
        char ch;
        public Calculator(String input)
        {
            this.input = input;
            szArr = input.ToCharArray();
           length = szArr.Length;
        }
        char nextToken()
        {
            ++current;
            if(current==length) return 'p';
            return szArr[current];
        }
        public double expression()
        {
            double d = term();
            while(true){
            switch (ch)
            {
                case '+':
                    d+= term();
                    break;
                case  '-':
                    d -= term();
                    break;
                default:
                    break;

            }
            if (ch == 'p') break;
        }
            return d;

        }


        public double term()
        {
            double d = primary();
            switch (ch)
            {
                case '*':
                    d *= primary();
                    break;
                case '/':
                     d /= primary();
                    break;
                default:
                    break;

            }
            return d;
        }
        public double primary()
        {
            string str = "";
            double value = 0.0;
            while (true)
            {

                ch = nextToken();
                if (isDigit(ch))
                {
                    str += ch;
                }
                else
                {
                    break;
                }
            }
            value = double.Parse(str);
            return value;

        }

        bool isDigit(char ch)
        {
            return ch == '0' || ch == '1' || ch == '2' || ch == '3' || ch == '4' || ch == '5' || ch == '6' || ch == '7' || ch == '8' || ch == '9';
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Enter expression");
            string input = Console.ReadLine();
            Console.WriteLine(new Calculator(input).expression());          
        }
    }
}

Source: stackoverflow.com

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