SherasAddNumber, C#

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using System;
namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string str1 = "123123123123123123213123",
                str2 = "212321312312312312312312312321312312312312",
                strResult;
            StrLongInteger v1 = new StrLongInteger(str1);
            StrLongInteger v2 = new StrLongInteger(str2);
            strResult = (v1 + v2).ToString();
            Console.WriteLine("Sum of Two number:" + strResult);
        }
    }
    public class StrLongInteger
    {
        private string strNumber;  
        public StrLongInteger(string numberAsString)
        {
            this.strNumber = numberAsString;
        }
        // Adds two string long numbers.
        public static StrLongInteger operator +(StrLongInteger strLI1, StrLongInteger strLI2)
        {
            char[] outputChars = new char[500];
            string longStr1 = Reverse(strLI1.ToString());
            string longStr2 = Reverse(strLI2.ToString());
            int length1 = longStr1.Length;
            int length2 = longStr2.Length;
            int maxLength = length1 > length2 ? length1 : length2;
            int carry = 0;
            int j = 0;
            for (j = 0; j < maxLength; j++)
            {
                int digit1 = (j > length1 - 1) ? 0 : Convert.ToInt32(longStr1[j].ToString());
                int digit2 = (j > length2 - 1) ? 0 : Convert.ToInt32(longStr2[j].ToString());
                int digitSum = digit1 + digit2 + carry;
                if (digitSum >= 10)
                {
                    digitSum -= 10;
                    carry = 1;
                }
                else
                    carry = 0;
                outputChars[j] = Convert.ToChar(Convert.ToString(digitSum));
            }
            if (carry == 1)
                outputChars[j++] = '1';
            return new StrLongInteger(Reverse(new string(outputChars)));
        }
        // reverses the string.
        private static string Reverse(string strNumber)
        {
            int j = 0;
            char[] outputChars = new char[strNumber.Length];
            for (int i = strNumber.Length - 1; i >= 0; i--)
            {
                outputChars[j] = strNumber[i];
                j++;
            }
            return new string(outputChars);
        }
        public override string ToString()
        {
            return strNumber.TrimStart('\0');
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,14 sec, absolute running time: 0,08 sec, cpu time: 0,08 sec, average memory usage: 13 Mb, average nr of threads: 3