Dear ALL:
   I'm writing a program about printer by C# now.I have read Duncan's
article about monitor printer by VB before and followed his method. I use
FindFirstPrinterChangeNotification and FindNextPrinterChangeNotification API
to monitor printer.
   Now I'm puzzled by the FindNextPrinterChangeNotification function
.Whenever I call this method to get a pointer of PRINTER_NOTIFY_INFO , it
always return null.
   My code is:

 int phand;
 int whand;
 RegisteredWaitHandle rwh;
 ManualResetEvent wh;

 OpenPrinter("MicrosoftOffice",ref phand, 0) ; //MicrosoftOffice is my
printer name
 whand =
FindFirstPrinterChangeNotification(phand,(int)PrinterChangeNotificationGener
alFlags.PRINTER_CHANGE_JOB,0,0);
 wh = new ManualResetEvent(false);
 wh.Handle = new IntPtr(whand);
 rwh = ThreadPool.RegisterWaitForSingleObject(wh,new
WaitOrTimerCallback(PrinterNotifyWaitCallBack),wh,-1,true);

 public void PrinterNotifyWaitCallBack(object state,bool timeout)
 {
  int pdwChange=0;
  IntPtr ptr= IntPtr.Zero;

  bool next = FindNextPrinterChangeNotification(whand,ref pdwChange,0,ref
ptr); //The next is true,but the ptr always returns 0
  PRINTER_NOTIFY_INFO pni =
(PRINTER_NOTIFY_INFO)Marshal.PtrToStructure(ptr,typeof(PRINTER_NOTIFY_INFO))
; //So the pni is always null.

  FreePrinterNotifyInfo(ptr);
  MessageBox.Show("Notify");
  wh.Reset();
  rwh = ThreadPool.RegisterWaitForSingleObject(wh,new
WaitOrTimerCallback(PrinterNotifyWaitCallBack),wh,-1,true);
 }

   I know here are many experts at this field.Could anyone tell me why and
how to solve this problem? I'm crashed by it.

   Thank you very much!