string path = @"c:\a\b\c\a.txt ";
            FileInfo fi = new FileInfo(path);
            DirectoryInfo di = fi.Directory;
            MessageBox.Show(di.Name);

------解决方案--------------------
 string s = @"c:\a\b\c\a.txt ";

  string[] dir = System.IO.Path.GetDirectoryName(s).Split(System.IO.Path.DirectorySeparatorChar);

  MessageBox.Show(dir[dir.Length - 1]);