Virtual printer port monitor installing

Question

I have a port monitor dll, that I instaling by call AddMonitor function of the spooler. But when I want uninstal this monitor, the DeleteMonitor function return errorcode 3008 - "The specified print monitor is currently in use". How I can free my monitor dll?

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
    private class MONITOR_INFO_2
    {
        [MarshalAs(UnmanagedType.LPStr)]
        public string pName;
        [MarshalAs(UnmanagedType.LPStr)]
        public string pEnvironment;
        [MarshalAs(UnmanagedType.LPStr)]
        public string pDLLName;
    }

    [DllImport("winspool.Drv", EntryPoint = "AddMonitorA", SetLastError = true, ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
    private static extern bool AddMonitor(
    [MarshalAs(UnmanagedType.LPStr)] string Name,
    Int32 Level,
    [In, MarshalAs(UnmanagedType.LPStruct)] MONITOR_INFO_2 mi2);

    [DllImport("winspool.Drv", EntryPoint = "DeleteMonitorA", SetLastError = true,  ExactSpelling = true, CallingConvention = CallingConvention.StdCall)]
    private static extern bool DeleteMonitor(
    [MarshalAs(UnmanagedType.LPStr)] string pNullServerName,
    [MarshalAs(UnmanagedType.LPStr)] string pNullEnvironment,
    [MarshalAs(UnmanagedType.LPStr)] string MonitorName);

    private unsafe void InstallMonitor(string monitorName, string dllName)
    {
        MONITOR_INFO_2 mi2 = new MONITOR_INFO_2();
        mi2.pName = monitorName;
        mi2.pEnvironment = null;
        mi2.pDLLName = dllName;

        try
        {
            bool bRet = AddMonitor(null, 2, mi2);
            if (!bRet)
                throw new Win32Exception(Marshal.GetLastWin32Error());
        }
        catch (Exception e)
        {
            if (!DeleteMonitor(null, null, monitorName))
            {
                throw new Win32Exception(Marshal.GetLastWin32Error());
            }
            bRet = AddMonitor(null, 2, mi2);
            if (!bRet)
                throw new Win32Exception(Marshal.GetLastWin32Error());
        }
    }

Answer 1

You will not be able to Delete a Port Monitor via the DeleteMonitor call if there is one or more printer objects currently using a port of that type.

This leaves you with several options:

• Swap the port of all affected printers to another port. (Best to use something like LPT1: since its always there).
• Delete all printers using the port.
• Stop the spooler service and remove the appropriate entries from registry (HKLM\SYSTEM\CurrentControlSet\Control\Print\Monitors) then restart the spooler. This will leave the affected printers there but they will be unusable.