SHChangeNotifyRegister（）注册了一个消息监控文件夹被删除的消息

我在一个对话框中做了一下工作，我在SHChangeNotifyRegister（）注册了一个消息函数
OnMediaChanged（）
HWND hWnd = GetSafeHwnd();
LPITEMIDLIST ppidl;
if(SHGetSpecialFolderLocation(hWnd, CSIDL_DESKTOP, &ppidl) == NOERROR)
{
SHChangeNotifyEntry shCNE = {0};
shCNE.pidl = ppidl;
shCNE.fRecursive = TRUE;

m_ulSHChangeNotifyRegister = SHChangeNotifyRegister
(hWnd,

SHCNRF_InterruptLevel |SHCNRF_ShellLevel

SHCNE_RMDIR ,

WM_USER_MEDIACHANGED,

1,

&shCNE);

ASSERT(m_ulSHChangeNotifyRegister != 0);

}

else
ASSERT(FALSE);

下面是消息函数OnMediaChanged（）的实现部分
LRESULT CCsDlg::OnMediaChanged(WPARAM wParam, LPARAM lParam)
{
SHNOTIFYSTRUCT *shns = (SHNOTIFYSTRUCT *)wParam;
CString strPath;
CString strMsg;

switch(lParam)
{

case SHCNE_DELETE://文件被删除
{
strPath = GetPathFromPIDL(shns->dwItem1);
if (!strPath.IsEmpty())
{
if (strPath != strMsg)
{
strMsg = strPath;
AfxMessageBox(strPath);
}

}
}
break;
}
监控不到同时删除多于4个的文件夹消息，一个被删除文件夹的路径也得不到

请高手指点 急急急 ？？？？？？？？？