窗体被另一个窗体覆盖，怎么才能抓取该窗体截图

窗体被另一个窗体覆盖，如何才能抓取该窗体截图？
我的代码，只能截取最上层的窗体。
[code=C/C++][/code]

  Graphics::TBitmap *scrBitmap = new Graphics::TBitmap();

  scrBitmap->Width = frmMain->Width;

  scrBitmap->Height = frmMain->Height;

  HDC hdc=GetWindowDC(0);

  BitBlt(scrBitmap->Canvas->Handle,0,0,scrBitmap->Width,scrBitmap->Height,hdc,frmMain->Left,frmMain->Top,SRCCOPY);

  scrBitmap->SaveToFile("D:\\1.bmp");

  delete scrBitmap;

[解决办法]

SendMessage(Form2->Handle,WM_PAINT,(WPARAM)Canvas->Handle,0);
[解决办法]