OnChangedExample.cs

using System;
using System.IO;

namespace ConsoleApp
{

    public class SomeService
    {
        private FileSystemWatcher _watcher;
        public void OnStart()
        {
            _watcher = new FileSystemWatcher();
            _watcher.Path = @"c:\temp";
            _watcher.NotifyFilter = NotifyFilters.LastWrite;
            _watcher.Filter = "*.*";
            _watcher.Changed += new FileSystemEventHandler(OnChanged);
            _watcher.EnableRaisingEvents = true;
        }

        private void OnChanged(object source, FileSystemEventArgs e)
        {
            Console.WriteLine(e.Name);
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            var thing = new SomeService();
            thing.OnStart();
            Console.ReadLine();
        }
    }
}