C# 怎么判断文件夹里的文件已经发生改变

C# 如何判断文件夹里的文件已经发生改变
C# 如何判断文件夹里的文件已经发生改变（比如判断不定时生成的日志文件）

[解决办法]

C# code

using System;using System.IO;using System.Security.Permissions;public class Watcher{    public static void Main()    {    Run();    }    [PermissionSet(SecurityAction.Demand, Name="FullTrust")]    public static void Run()    {        string[] args = System.Environment.GetCommandLineArgs();        // If a directory is not specified, exit program.        if(args.Length != 2)        {            // Display the proper way to call the program.            Console.WriteLine("Usage: Watcher.exe (directory)");            return;        }        // Create a new FileSystemWatcher and set its properties.        FileSystemWatcher watcher = new FileSystemWatcher();        watcher.Path = args[1];        /* Watch for changes in LastAccess and LastWrite times, and           the renaming of files or directories. */        watcher.NotifyFilter = NotifyFilters.LastAccess | NotifyFilters.LastWrite           | NotifyFilters.FileName | NotifyFilters.DirectoryName;        // Only watch text files.        watcher.Filter = "*.txt";        // Add event handlers.        watcher.Changed += new FileSystemEventHandler(OnChanged);        watcher.Created += new FileSystemEventHandler(OnChanged);        watcher.Deleted += new FileSystemEventHandler(OnChanged);        watcher.Renamed += new RenamedEventHandler(OnRenamed);        // Begin watching.        watcher.EnableRaisingEvents = true;        // Wait for the user to quit the program.        Console.WriteLine("Press \'q\' to quit the sample.");        while(Console.Read()!='q');    }    // Define the event handlers.    private static void OnChanged(object source, FileSystemEventArgs e)    {        // Specify what is done when a file is changed, created, or deleted.       Console.WriteLine("File: " +  e.FullPath + " " + e.ChangeType);    }    private static void OnRenamed(object source, RenamedEventArgs e)    {        // Specify what is done when a file is renamed.        Console.WriteLine("File: {0} renamed to {1}", e.OldFullPath, e.FullPath);    }}