public static class SprechendeZahlen
    {
        private static string[] kleiner20 = new string[] { "null", "ein", "zwei", "drei", "vier", "fünf", "sechs", "sieben", "acht", "neun", "zehn", "elf", "zwölf", "dreizehn", "vierzhen", "fünfzehn", "sechszehn", "siebzehn", "achtzehn", "neunzehn" };
        private static string[] zehner = new string[] { "null", "zehn", "zwanzig", "dreißig", "vierzig", "fünfzig", "sechzig", "siebzig", "achtzig", "neunzig" };
        private static string[] singular = new string[] { "s", "tausend", "e Million", "e Milliarde", "e Billion", "e Billiarde", "e Trillion", "e Trilliade" };
        private static string[] plural = new string[] { "", "tausend", "Millionen", "Milliarden", "Billionen", "Billiarden", "Trillionen", "Trilliaden", "Quadrillionen", "Quadrilliarden" };

        public static string GetZahlenname(int zahl)
        {
            if (zahl == 0)
                return "Null";
            else if (zahl < 1 || zahl ≥  Math.Pow(plural.Length, 28))
                return "InvalidNumber";
            List<int> gruppe = new List<int>();
            int i = -1, stellen;
            while (zahl>0)
            {
                stellen = zahl % 1000;
                zahl = zahl / 1000;
                i++;
                gruppe.Add(stellen);
            }

            StringBuilder sb = new StringBuilder();
            while (i≥0)
            {
                sb.Append(GetHunderter(gruppe[i], singular[i], plural[i]));
                i--;
            }
            return sb.ToString();
        }

        private static string GetHunderter(int zahl, string singular, string plural)
        {
            if (zahl == 0)
                return "";
            else if (zahl == 1)
                return "ein" + singular;
            StringBuilder sb = new StringBuilder();

            var h = zahl / 100;
            var r = zahl % 100;
            if (h > 0)
                sb.Append(kleiner20[h] + "hundert");
            if (r>0)
            {
                if (r == 1)
                    sb.Append("eins");
                else if (r < 20)
                    sb.Append(kleiner20[r]);
                else
                {
                    var z = r / 10;
                    var rr = r % 10;
                    if (rr > 0)
                        sb.Append(kleiner20[rr]);
                    if (rr > 0 && z > 0)
                        sb.Append("und");
                    if (z > 0)
                        sb.Append(zehner[z]);
                }
            }
            return sb.ToString()+plural;
        }
    }