Monitoring a print queue from Visual Basic.Net

To avoid blocking the UI thread, you register the printer‑change event with the thread pool. The call looks like this:

When the spooler signals the event, the thread‑pool thread calls the PrinterNotifyCallback routine. The callback receives a state object and a Boolean indicating whether the event fired or a timeout occurred. Inside the callback you must perform the following actions:

• Check the pdwChange value returned by FindNextPrinterChangeNotification to determine the exact type of event (e.g. PRINTER_CHANGE_ADD_JOB).
• Retrieve the pointer to the PRINTER_NOTIFY_INFO data structure. The spooler allocates this buffer, and you free it with LocalFree after you are done.
• Marshal the data into managed objects for easy consumption.

  The callback must be quick – the thread‑pool thread is part of the global pool, and holding it for long can starve other threads. A common pattern is to enqueue a work item or post a message to the UI thread so that heavy processing happens elsewhere.

  Handling a Notification and Extracting Job Information

  Inside PrinterNotifyCallback you first call FindNextPrinterChangeNotification with the printer handle and the same mask you used earlier. The function returns a 32‑bit flag set in pdwChange and a pointer to a PRINTER_NOTIFY_INFO buffer. The buffer contains a header followed by an array of PRINTER_NOTIFY_INFO_DATA structures, each holding one piece of data you requested.

  Prompt

  Dim changeFlags As UInt32</p> <p>Dim notifyInfoPtr As IntPtr</p> <p>Dim result As Boolean = FindNextPrinterChangeNotification(</p> <p> printerHandle,</p> <p> changeMask,</p> <p> 0,</p> <p> changeFlags,</p> <p> notifyInfoPtr</p> <p>)</p> <p>If result Then</p> <p> Dim notifyInfo As PRINTER_NOTIFY_INFO = Marshal.PtrToStructure(Of PRINTER_NOTIFY_INFO)(notifyInfoPtr)</p> <p> For i As Integer = 0 To notifyInfo.cData - 1</p> <p> Dim data As PRINTER_NOTIFY_INFO_DATA = Marshal.PtrToStructure(Of PRINTER_NOTIFY_INFO_DATA)(</p> <p> Marshal.ReadIntPtr(notifyInfoPtr, Marshal.SizeOf(notifyInfo) + i * IntPtr.Size)</p> <p> )</p> <p> ' Process data based on notifyInfo.pszDataType</p> <p> Next</p> <p> LocalFree(notifyInfoPtr)</p> <p>End If</p>

  The pszDataType field tells you which data type the current PRINTER_NOTIFY_INFO_DATA contains – job ID, job title, user name, page count, etc. You can map these numeric codes to meaningful names in a dictionary for readability. The data itself is stored in the data union. For numeric types you can read the ul field; for strings you read the psz pointer and call Marshal.PtrToStringUni to convert to a .NET string.

  For example, if you requested the job ID and user name, the loop might look like this:

  Prompt

  Dim jobId As UInt32 = 0</p> <p>Dim userName As String = ""</p> <p>For i = 0 To notifyInfo.cData - 1</p> <p> Dim infoData As PRINTER_NOTIFY_INFO_DATA = Marshal.PtrToStructure(Of PRINTER_NOTIFY_INFO_DATA)(</p> <p> Marshal.ReadIntPtr(notifyInfoPtr, Marshal.SizeOf(notifyInfo) + i * IntPtr.Size)</p> <p> )</p> <p> Select Case infoData.pszDataType</p> <p> Case 1 ' JOB_ID</p> <p> jobId = infoData.ul</p> <p> Case 2 ' USER_NAME</p> <p> userName = Marshal.PtrToStringUni(infoData.psz)</p> <p> End Select</p> <p>Next</p>

  After you have extracted the information you need, you can update a UI element, log to a file, or trigger any other business logic. Because the spooler may return multiple data blocks in a single event, you should iterate until you collect all required fields.

  Once you finish with the handle returned by FindFirstPrinterChangeNotification, call FindClosePrinterChangeNotification to let the spooler clean up its internal event object. Finally, when the application exits, release the printer handle with ClosePrinter and free any unmanaged memory you allocated for the options structure.

  With these pieces in place you have a robust, low‑overhead listener that reacts instantly to changes in the print queue. It works on Windows NT, 2000, XP and all .NET server releases, but not on legacy Windows 9x platforms. If you run into any quirks or need help tailoring the code to a specific printer model, feel free to reach out to the development team.