Exception with factorial example demo in C#

Below is the code which calculates the factorial of a number. If we pass a negative number as an argument it will throw a user defined exception. For this we have created a class which inherits an "exception" superclass and it contains an overriden tostring() method to print the user defined exception message.


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

namespace Exception_inheritance_demo
{
    class MyEx : Exception
    {
        public override string ToString()
        {
            return "No not allowed";
        }
     
    }
    class A
    {
        void facto(int n)
        {
            if (n < 0)
                throw new MyEx();
            else
            {
                int f = 1;
                while (n > 0)
                {
                    f = f * n;
                    n--;
                }

                Console.WriteLine("Factorial="+ f.ToString());
            }
        }

        static void Main(string[] args)
        {
            A a1 = new A();
            try
            {
                a1.facto(4);
                a1.facto(-4);
                a1.facto(3);
            }catch(Exception ex)
            {
                Console.WriteLine("{0}", ex.ToString());
            }
            Console.ReadLine();
        }
    }
 
}