如何直接修改打印机设置(如纸张大小和打印方向等),写了一段但不行不知道错在那里,请大家帮我看看!急!

本人代码如下:
  Device:array[0..255] of Char;
  Driver:array[0..255] of Char;
  Port:array[0..255] of Char;
  hDeviceMode:THandle;
  pDeviceMode:PDevMode;
begin
  Printer.PrinterIndex:=Printer.PrinterIndex;
  Printer.GetPrinter(Device,Driver,Port,hDeviceMode);
  pDeviceMode:=GlobalLock(hDeviceMode);
  pDeviceMode.dmPaperSize:=DMPAPER_A4;
  pDeviceMode.dmOrientation:=DMDUP_VERTICAL;
  GlobalUnLock(hDeviceMode);
end;
程序编译通过，手动把打印机设置改为A5和横向打印后，运行程序后还是原设置，没有改为A4和纵向打印。请各位高手帮我看看好吗？
很急！希望大家帮忙噢！