Why CDC::LineTo() doesn't draw in Visual C++ 2015 MFC Dialog? - c++

Question

I'm learning MFC and I'm trying to draw some lines on a MFC Dialog-based application main window, it shall be a rather simple task but while running I see no lines drawing on the dialog.
Following is the method I wrote:
// draw corner of a rectangle on specified device context
void CTestDrawCornerDlg::DrawCorner(
CDC* pDC,
const CornerType& type,
const CPoint& position,
const unsigned int& size
)
{
CPen pen(PS_SOLID, 5, RGB(0, 0, 0));
CPen* pOldPen = pDC->SelectObject(&pen);
CPoint pH, pV;
// I could make following lines simply with a 2-lines block,
// but I'd leave it as it was to make it easier to understand.
switch (type)
{
case LEFT_TOP:
pH.x = position.x + size;
pH.y = position.y;
pV.x = position.x;
pV.y = position.y + size;
break;
case LEFT_BOTTOM:
pH.x = position.x - size;
pH.y = position.y;
pV.x = position.x;
pV.y = position.y + size;
break;
case RIGHT_TOP:
pH.x = position.x + size;
pH.y = position.y;
pV.x = position.x;
pV.y = position.y - size;
break;
case RIGHT_BOTTOM:
pH.x = position.x - size;
pH.y = position.y;
pV.x = position.x;
pV.y = position.y - size;
break;
default: break;
}
pDC->MoveTo(position);
pDC->LineTo(pH);
pDC->MoveTo(position);
pDC->LineTo(pV);
pDC->SelectObject(pOldPen);
}
And I called this method in OnPaint method of Dialog class:
void CTestDrawCornerDlg::OnPaint()
{
if (IsIconic())
{
CPaintDC dc(this); // device context for painting
// lines generated automatically when creating
// MFC project are truncated for brevity
}
else
{
CDialogEx::OnPaint();
}
CPaintDC pDC(this);
DrawCorner(&pDC, LEFT_TOP, CPoint(50, 50), 50);
}
I guess it's a newbie mistake but I just don't know what the mistake is. Thanks for help!
P.S. please download from following link the MFC project to re-create this problem:
https://www.dropbox.com/s/exeehci9kopvgsn/TestDrawCorner.zip?dl=0

Answer 1

You can change your code to use CDialogEx::OnPaint() + CClientDC as follows:
void CTestDrawCornerDlg::OnPaint()
{
CDialogEx::OnPaint();
CClientDC pDC(this);
DrawCorner(&pDC, LEFT_TOP, CPoint(50, 50), 50);
}
or just use CPaintDC:
void CTestDrawCornerDlg::OnPaint()
{
CPaintDC pDC(this);
DrawCorner(&pDC, LEFT_TOP, CPoint(50, 50), 50);
}
But don't use OnPaint + CPaintDC
To see the problem, note how OnPaint and CPaintDC are defined in MFC:
void CDialog::OnPaint()
{
CPaintDC dc(this);
if (PaintWindowlessControls(&dc))
return;
Default();
}
CPaintDC::CPaintDC(CWnd* pWnd)
{
if (!Attach(::BeginPaint(m_hWnd = pWnd->m_hWnd, &m_ps)))
AfxThrowResourceException();
}
::BeginPaint is a core WinAPI function. It should only be called once in response to WM_PAINT, and it can't be used anywhere else.
CClientDC on the other hand uses ::GetDC which can be used pretty much anywhere, as long as window handle is available.