这个是看别人的代码得到的，不是我写的。

void CHhDlg::OnOK()
{
  //先得到屏幕的长宽：
  int ScreenWidth = ::GetSystemMetrics(SM_CXSCREEN);
  int ScreenHeight = ::GetSystemMetrics(SM_CYSCREEN);
  HDC hDC = ::GetDC(0);
  //得到“画布(Canvas)”
  CDC* ScreenCanvas = new CDC();
  ScreenCanvas->m_hDC = hDC;
  //设置画布的画笔(Pen)
  CPen pen(1, 10, RGB(255,0,0));
  CPen* oldpen;
  oldpen=ScreenCanvas->SelectObject(&pen);
  //ScreenCanvas->Pen->Color = clRed;  //画笔的颜色为红色(Red)
  //ScreenCanvas->Pen->Width = 10;     //画笔的粗细
  //开始画叉啦，需要画两笔,这是第一笔：左上到右下
  ScreenCanvas->MoveTo(0,0);        //把画笔移到(Move to)坐标0，0处，即屏幕最左上角
  ScreenCanvas->LineTo(ScreenWidth,ScreenHeight);  //从当前位置画一条线(Line to)到屏幕的右下角。

  //第二笔：右上到左下：
  ScreenCanvas->MoveTo(ScreenWidth,0);
  ScreenCanvas->LineTo(0,ScreenHeight);

  ScreenCanvas->SelectObject(oldpen);
  //释放“画布”:
 
  ScreenCanvas->m_hDC = 0;
 
  delete ScreenCanvas;
  ::ReleaseDC(0,hDC);

  //CDialog::OnOK();
}