using System;

​

public class Functions

{

    public static long Factorial(int n)

    {

        // Test for invalid input.

        if ((n < 0) || (n > 20))

        {

            return -1;

        }

​

        // Calculate the factorial iteratively rather than recursively.

        long tempResult = 1;

        for (int i = 1; i <= n; i++)

        {

            tempResult *= i;

        }

        return tempResult;

    }

}

​

public class MainClass

{

    public static int Main(string[] args)

    {

        // Test if input arguments were supplied.

        if (args==null)

        {

            Console.WriteLine("Please enter a numeric argument.");

            Console.WriteLine("Usage: Factorial <num>");

            return 1;

        }

​

        // Try to convert the input arguments to numbers. This will throw

        // an exception if the argument is not a number.

        // num = int.Parse(args[0]);

        int num;

        bool test = int.TryParse(args[0], out num);

        if (!test)

        {

            Console.WriteLine("Please enter a numeric argument.");

            Console.WriteLine("Usage: Factorial <num>");

            return 1;

        }

​

        // Calculate factorial.

        long result = Functions.Factorial(num);

​

        // Print result.

        if (result == -1)

            Console.WriteLine("Input must be >= 0 and <= 20.");

        else

            Console.WriteLine("The Factorial of {num} is {result}.");

​

        return 0;

    }

}