sum two strings without casting

Dim a, b, c As String
a = "1"
b = "2"
c = 0 + a + b
Response.Write("c=" + C)

static class Program
    {
        static void Main()
        {
            const string s = "1234566633434";
            const string s2 = "8934343433";
            //Console.WriteLine(long.Parse(s) + long.Parse(s2));
            Console.WriteLine(ToLong(s) + ToLong(s2));
        }



        private static long ToLong(string str)
        {
            long num = 0;
            var sign = false;
            if (IsDigit(str))
            {
                if (str[0] == '-') sign = true;

                num = str
                    .Where(q => q != '-')
                    .Aggregate(num, (current, item) => current * 10 + item - '0');
            }
            return sign ? -num : num;
        }

        private static bool IsDigit(string str)
        {
            return Regex.IsMatch(str, @"^-?\d+$");
        }

    }