近來發現了一個效率更高的方法，只需遍歷目錄就能獲得檔案個數

System.IO.DirectoryInfo dirInfo = new System.IO.DirectoryInfo(dirPath); 
public static int GetFilesCount(System.IO.DirectoryInfo dirInfo) 
{ 
    int totalFile = 0; 
    totalFile += dirInfo.GetFiles().Length; 
    foreach (System.IO.DirectoryInfo subdir in dirInfo.GetDirectories
 
()) 
    { 
        totalFile += GetFilesCount(subdir); 
    } 
    return totalFile; 
}