Factorial.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Factorial
{
    class Program
    {
        static void Main(string[] args)
        {
            double input, output;
            string text;
            while (true)
            {
                Console.WriteLine("Enter a number: ");
                text = Console.ReadLine();
                if (double.TryParse(text, out input))
                {
                    output = 1f;
                    for (double i = input; i > 0; i--)
                    {
                        output *= i;
                    }
                    Console.WriteLine("Factorial of " + input + ": " + output);
                }
                else
                {
                    Console.WriteLine("error");
                }
            }
        }
    }
}