PZ

 // Relevant parts of code of GFMAPI.NET for FMOpen and FMClose (www.netripper.nl)

    public enum ResultCode : int
    {
        Success                 = 0,
        InvalidParameter        = 87,
        CallNotImplemented      = 120,
        NotSupported            = 50,
        BadUnit                 = 20,
        NotReady                = 21,
        OpenFailed              = 110,
        DeviceAlreadyAttached   = 548,
        GenFailure              = 31
    }

 // ...snip...

        private const string DLL = "GFMAPI.DLL";

        [DllImport(DLL, SetLastError = true)]
        public static extern IntPtr FMOpen(int FMDeviceIndex);

        [DllImport(DLL)]
        public static extern ResultCode FMClose(IntPtr FMDevice);

 // ...in the calling class when FMOpen is called (in my test app)...

                    // Opening device
                    FMDevice = GFMAPI.FMOpen(0);

                    // Fetching device information
                    TagFMDEVINFO devinfo = new TagFMDEVINFO();
                    devinfo.Size = Marshal.SizeOf(devinfo);
                    ResultCode rc = GFMAPI.FMGetDevInfo(0, ref devinfo);

                    // Register window for notifications
                    ResultCode result = GFMAPI.FMRegisterNotification(FMDevice, MessageWindow.Hwnd, GFMAPI.WM_FMNOTIFICATION);

 // ... in the calling class when FMClose is called...

                    // Closing device
                    ResultCode result = GFMAPI.FMClose(FMDevice);