static void Main(string[] args)
        {
            Console.Write("Fakultätberechner von Jakob Horak\n");

            string val, val0;
            bool finish;

            do
            {
                do
                {
                    Console.Write("\nGeben sie bitte den Wert ein, von dem\ndie Fakultät berechnet werden soll: ");
                } while (Read(out val));

                val0 = val;

                if (val == "0")
                    Console.Write("Die Fakultät von 0 beträt 1\n");
                else
                {
                    Console.WriteLine("Berechnen...");
                    StringFactorialref(ref val);
                    Console.Write("Die Fakultät von {0} beträt {1}\n", val0, val);
                }
                Console.Write("Wollen Sie das Programm wiederholen? (Y/N): ");
            } while (Console.ReadLine().ToUpper() == "Y");
        }

        static bool Read(out string value)
        {
            int temp;
            if (!int.TryParse(Console.ReadLine(), out temp) || temp < 0)
            {
                Console.WriteLine("Ungültige Eingabe!");
                value = "0";
                return true;
            }
            else
                value = temp.ToString();
            return false;
        }

        static void StringFactorialref(ref string value)
        {
            string temp = "1";
            for (string i = "1"; StringGoE(value, i); i = StringAdd(i, "1"))
            {
                temp = StringMulti(temp, i.ToString());
            }

            value = temp;
        }

        //static string StringFactorial(string value)
        //{
        //    if (value == "0")
        //        return "1";
        //    else
        //        return StringMulti(StringFactorial(StringDecrease(value)), value);
        //}

        static string StringDecrease(string value)
        {
            char[] arr = value.ToCharArray();
            Array.Reverse(arr);
            for (int i = arr.Length - 1; i >= 0; i--)
            {
                if (arr[i] == '0')
                    continue;
                else
                {
                    arr[i] = Convert.ToChar(Convert.ToString(Convert.ToInt32(arr[i].ToString()) - 1));
                    break;
                }
            }
            return new string(arr);
        }

        static string StringAdd(string a, string b)
        {
            char[] arra = a.ToCharArray(), arrb = b.ToCharArray(), temp, output;
            int i, tempint;
            int[] intarr;
            string stringout;

            Array.Reverse(arra);
            Array.Reverse(arrb);

            if (arra.Length < arrb.Length)
            {
                temp = arra;
                arra = arrb;
                arrb = temp;
            }

            if (arra.Length != arrb.Length)
            {
                temp = new char[arra.Length];
                for (i = 0; i < arrb.Length; i++)
                    temp[i] = arrb[i];
                arrb = temp;
            }

            intarr = new int[arra.Length + 1];


            for (i = 0; i < arra.Length; i++)
            {
                int.TryParse(arra[i].ToString(), out tempint);
                intarr[i] = tempint;
                int.TryParse(arrb[i].ToString(), out tempint);
                intarr[i] += tempint;
            }

            bool carry = false;
            output = new char[intarr.Length];

            for (i = 0; i < intarr.Length; i++)
            {
                if (carry)
                    intarr[i] += 1;
                carry = intarr[i] >= 10;
                output[i] = Convert.ToChar(Convert.ToString(intarr[i] % 10));
            }

            Array.Reverse(output);
            stringout = new string(output);
            while (stringout[0] == '0')
                stringout = stringout.Substring(1);

            return stringout;
        }

        static string StringMulti(string a, string b)
        {
            string temp = "0";
            for (string i = "1"; StringGoE(b, i); i = StringAdd(i, "1"))
            {
                temp = StringAdd(temp, a);
            }
            return temp;
        }

        static bool StringGoE(string a, string b)
        {
            while (a[0] == '0')
                a = a.Substring(1);

            while (b[0] == '0')
                b = b.Substring(1);

            if (a.Length > b.Length)
                return true;
            else if (a.Length < b.Length)
                return false;
            else
            {
                for (int i = a.Length - 1; i >= 0; i--)
                {
                    if (Convert.ToInt32(a[i]) > Convert.ToInt32(b[i]))
                        return true;
                    if (Convert.ToInt32(a[i]) < Convert.ToInt32(b[i]))
                        return false;
                }
                return true;
            }
        }

        //static string StringAdd(string[] arr)
        //{
        //    string temp = StringAdd(arr[0], arr[1]);
        //    for (int i = 2; i < arr.Length; i++)
        //    {
        //        temp = StringAdd(temp, arr[i]);
        //    }
        //    return temp;
        //}