Code:
public class Functions
{
    public static long Factorial(int n)
    {
        if (n < 0) { return -1; }    //error result - undefined
        if (n > 256) { return -2; }  //error result - input is too big

        if (n == 0) { return 1; }

        // Calculate the factorial iteratively rather than recursively:

        long tempResult = 1;
        for (int i = 1; i <= n; i++)
        {
            tempResult *= i;
        }
        return tempResult;
    }
}

class MainClass
{
    static int Main(string[] args)
    {
        // Test if input arguments were supplied:
        if (args.Length == 0)
        {
            System.Console.WriteLine("Please enter a numeric argument.");
            System.Console.WriteLine("Usage: Factorial <num>");
            return 1;
        }

        try
        {
            // Convert the input arguments to numbers:
            int num = int.Parse(args[0]);

            System.Console.WriteLine("The Factorial of {0} is {1}.", num, Functions.Factorial(num));
            return 0;
        }
        catch (System.FormatException)
        {
            System.Console.WriteLine("Please enter a numeric argument.");
            System.Console.WriteLine("Usage: Factorial <num>");
            return 1;
        }
    }
}

output:
// arguments: 3
The Factorial of 3 is 6.