problem about FindNextPrinter ChangeNotificat ion

[sunny]

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
FindFirstPrinte rChangeNotifica tion and FindNextPrinter ChangeNotificat ion API
to monitor printer.
Now I'm puzzled by the FindNextPrinter ChangeNotificat ion 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;
RegisteredWaitH andle rwh;
ManualResetEven t wh;

OpenPrinter("Mi crosoftOffice", ref phand, 0) ; //MicrosoftOffice is my
printer name
whand =
FindFirstPrinte rChangeNotifica tion(phand,(int )PrinterChangeN otificationGene r
alFlags.PRINTER _CHANGE_JOB,0,0 );
wh = new ManualResetEven t(false);
wh.Handle = new IntPtr(whand);
rwh = ThreadPool.Regi sterWaitForSing leObject(wh,new
WaitOrTimerCall back(PrinterNot ifyWaitCallBack ),wh,-1,true);

public void PrinterNotifyWa itCallBack(obje ct state,bool timeout)
{
int pdwChange=0;
IntPtr ptr= IntPtr.Zero;

bool next = FindNextPrinter ChangeNotificat ion(whand,ref pdwChange,0,ref
ptr); //The next is true,but the ptr always returns 0
PRINTER_NOTIFY_ INFO pni =
(PRINTER_NOTIFY _INFO)Marshal.P trToStructure(p tr,typeof(PRINT ER_NOTIFY_INFO) )
; //So the pni is always null.

FreePrinterNoti fyInfo(ptr);
MessageBox.Show ("Notify");
wh.Reset();
rwh = ThreadPool.Regi sterWaitForSing leObject(wh,new
WaitOrTimerCall back(PrinterNot ifyWaitCallBack ),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!