Reverse String and Count sum whithout Parse to int

using System;
using System.Linq;
using System.Collections.Generic;

namespace SumOfTwoStrings
{
    class Program
    {
        static void Main(string[] args)
        {
            TestSum();
        }

        private static void TestSum()
        {
            string num1 = "123456";
            string num2 = "12";
            string result = CalculateSumWithoutParsing(num1, num2);
            Console.WriteLine(result);

            string num3 = "12";
            string num4 = "123456";
            string secondResult = CalculateSumWithoutParsing(num3, num4);
            Console.WriteLine(secondResult);

            string num5 = "123456";
            string num6 = "123456";
            string thirdResult = CalculateSumWithoutParsing(num5, num6);
            Console.WriteLine(thirdResult);

            string num7 = "123454";
            string num8 = "123454";
            string fourthResult = CalculateSumWithoutParsing(num7, num8);
            Console.WriteLine(fourthResult);

            string num9 = "0";
            string num10 = "0";
            string fifthResult = CalculateSumWithoutParsing(num9, num10);
            Console.WriteLine(fifthResult);
        }

        private static string CalculateSumWithoutParsing(string num1, string num2)
        {
            var firstStack = new Stack<char>(num1);
            var secondStack = new Stack<char>(num2);
            string result = string.Empty;
            int carry = 0;
            while (firstStack.Count > 0 && secondStack.Count > 0)
            {
                char currentDigitFromFirstStack = firstStack.Pop();
                char currentDigitFromSecondStack = secondStack.Pop();
                int currentSum = int.Parse(currentDigitFromFirstStack.ToString()) + int.Parse(currentDigitFromSecondStack.ToString());

                int newCurrentResult = currentSum + carry; // if already have carry added it
                carry = 0;
                if (newCurrentResult / 10 > 0) // check for the carry after newCurrentResult
                {
                    carry += newCurrentResult / 10;
                }

                result += (newCurrentResult % 10).ToString();
            }

            string finalResult = string.Empty;
            if (firstStack.Count == 0 && secondStack.Count > 0)
            {
                finalResult = GenereteFinalResult(secondStack, result, carry);
            }
            else if (secondStack.Count == 0 && firstStack.Count > 0)
            {
                finalResult = GenereteFinalResult(firstStack, result, carry);
            }
            else // the count of the both stecks is equal
            {
                finalResult = new String(result.ToCharArray().Reverse().ToArray());
            }

            return finalResult.Trim();
        }

        private static string GenereteFinalResult(Stack<char> secondStack, string result, int carry)
        {
            string finalResult;
            char currentDigitFromSecondStack = secondStack.Pop();
            int newCurrentResult = int.Parse(currentDigitFromSecondStack.ToString()) + carry;
            string temporaryResult = result + newCurrentResult;
            string reverseTemporaryResult = new String(temporaryResult.ToCharArray().Reverse().ToArray());
            finalResult = string.Join("", secondStack.ToArray().Reverse()) + reverseTemporaryResult; // how to reverse string in a one line
            return finalResult;
        }
    }
}