Thinking more...

用 foreach 以遞迴方式去找出所有檔案根目錄，順便練習 yield。

    class FileSystemWalker
    {
        private string _path = "";
        public FileSystemWalker(string path)
        {
            _path = path;
        }

        public IEnumerable<FileSystemInfo> Walk()
        {
            foreach( string d in Directory.GetDirectories( _path ) )
            {
                DirectoryInfo di = new DirectoryInfo( d );
                yield return di;
                FileSystemWalker walker = new FileSystemWalker(Path.Combine(_path, d));
                foreach (FileSystemInfo fsi in walker.Walk())
                    yield return fsi;
            }
            foreach (string f in Directory.GetFiles( _path ) )
            {
                FileInfo fi = new FileInfo(f);
                yield return fi;
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            foreach(FileSystemInfo fsi in new FileSystemWalker(@"f:\").Walk())
            {
                if( fsi.Attributes == FileAttributes.Directory )
                    Console.WriteLine( "[D]{0}", fsi.FullName );
                else
                    Console.WriteLine("[F]{0}", fsi.FullName);
            }
            Console.ReadLine();
        }
    }