factorial of a number

using System;

namespace Examples
{
    internal static class FactorialProgram
    {
        public static int Factorial(this int number)
        {
            if (number < 0) throw new ArgumentException("negative Zahl!");
            return number > 1 ? number * Factorial(--number) : 1;
        }

        public static void Main(string[] args)
        {
            Console.Write("Nicht-negative ganze Zahl: ");

            int value;
            if (!int.TryParse(Console.ReadLine(), out value)) return;

            Console.Write("{0}! = {1}", value, value.Factorial());
            Console.Read();
        }
    }
}