求一段能编译通过的int类型的拓展方法

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

namespace ConsoleTest
{
    static class intExtendMethod
    {
        public static void showmsg(this int i)
        {
            Console.WriteLine("int show extend message!");
        }

        public static int ReverseDigits(this int i)
        {
            /*************/
            /*degisn here*/
            /*************/
            return --i;  //return your result
        }

        public static int addOne(this int i,int n)
        {
            return i + n;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Program test = new Program();

            int i = 0;           
            test.testExtend(i, 1);

            i = 12345;
            test.testExtend(i, 10);

            i = -12345;
            test.testExtend(i, -10);

            Console.ReadKey();
        }

        private void testExtend(int m,int n)
        {
            m.showmsg();
            Console.WriteLine("{0}", m.ReverseDigits());
            Console.WriteLine("{0}", m.addOne(n));
        }
    }
}