Factorial.cs

// C# function to calculate the factorial of a given number.
using System.Linq;
using System.Collections.Generic;

using static System.Console;

public class Program
{
    public static void Main()
    {
        var numbers = 0.To(10);
        numbers.ForEach(x => WriteLine($"{x:00}! = {x.Fact()}"));
    }
}

public static class ExtensionsMethods
{
    public static long Fact(this long x) => x <= 1 ? 1 : x * Fact(x - 1);
	
    public static IEnumerable<long> To(this int st, int ct)
    {
	for (var x = st; x <= ct; x++)
	{
	    yield return x;
	}
    }

    public static void ForEach<T>(this IEnumerable<T> source, Action<T> action)
    {
	foreach (T item in source)
	    action(item);
    }
}

/* the output    
00! = 1                                                                                                                                                                                                                  
01! = 1                                                                                                                                                                                                                  
02! = 2                                                                                                                                                                                                                  
03! = 6                                                                                                                                                                                                                  
04! = 24                                                                                                                                                                                                                 
05! = 120                                                                                                                                                                                                                
06! = 720                                                                                                                                                                                                                
07! = 5040                                                                                                                                                                                                               
08! = 40320                                                                                                                                                                                                              
09! = 362880                                                                                                                                                                                                             
10! = 3628800
*/