public class Factorial
{

    public static void Main(string[] args)
    {
        string op;

        System.Console.Write("Input number : ");
        op = System.Console.ReadLine();
        System.Console.WriteLine(CalculateFactorial(int.Parse(op)));
    }

    public static int CalculateFactorial(int pNumber)
    {
        int result = 1;
        for (int i = 1; i <= pNumber; i++)
        {
            result = result * i;
        }
        return result;
    }

}

public class Factorial
{

    public static void Main(string[] args)
    {
        string op;

        System.Console.Write("Input number : ");
        op = System.Console.ReadLine();
        System.Console.WriteLine(CalculateFactorial(int.Parse(op)));
    }

    public static int CalculateFactorial(int pAngka)
    {
        int hasil = 1;
        for (int i = 1; i <= pAngka; i++)
        {
            hasil = hasil * i;
        }
        return hasil;
    }

}

Just compile in your command line and run

how to compile : csc Factorial.cs

Calculate Factorial

C#