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();
}
