Factorial is the product of an integer and all the integers below it; for example, factorial four (4!)=1*2*3*4=24.

C# program for Factorial.

using System;

namespace Factorial
{
    class Program
    {
        static void Main(string[] args)
        {
            int factorialNumber=1;
            Console.Write("Please enter a number:");
            int number = int.Parse(Console.ReadLine());
            for(int i=1; i<=number; i++)
            {
                factorialNumber = factorialNumber * i;
            }
            Console.Write($"Factorial number of {number} is {factorialNumber}");
        }
    }
}

Output:

Please enter a number:6
Factorial number of 6 is 720