C#

private double factorial(double f)
{
    double num = Math.Floor(f);
    if(num < 0)
    {
        return -1;
    }
    if(num == 0)
    {
        return 1;
    }
    else
    {
        return (num * factorial(num - 1));
    }
}

public static BigInteger factorial( int n )
{
   BigInteger result = new BigInteger(1);
   for( int i = 1; i <= n; i++ )
      result = result * i;
   return result;
}
... And later, in Main(): 
Console.WriteLine("factorial of 170  = " + factorial(170) );

range.htmlText = @"<span style=""color:white;background-color:darkblue;"">" + range.htmlText + @"</span>";