How to draw border in a non rectangular window in MFC - c++

Question

I have a mode less dialog that I have changed the shape into a roundrect using SetWindowRgn(). I would like to draw a colored border around it using FrameRgn. Here is the code I am using
BOOL CMyDlg::OnInitDialog()
{
CDialog::OnInitDialog(); m_Brush.CreateSolidBrush(RGB(255,255,255));
CRect rcDialog;
GetClientRect(rcDialog);
// This Creates area assigned to Dialog: This goes directly below the above in OnInitDialog
m_rgnShape.CreateRoundRectRgn(rcDialog.TopLeft().x, rcDialog.TopLeft().y,rcDialog.BottomRight().x,rcDialog.BottomRight().y, rcDialog.Width()/8, rcDialog.Height()/8);
::SetWindowRgn(GetSafeHwnd(), (HRGN)m_rgnShape, TRUE);
return TRUE; // return TRUE unless you set the focus to a control
// EXCEPTION: OCX Property Pages should return FALSE
}
void CMyDlg::OnPaint()
{
CPaintDC dc(this); // device context for painting
CBrush brush;
brush.CreateSolidBrush(RGB(255,0,0));
dc.FrameRgn(&m_rgnShape, &brush, 2, 2);
}
Can anyone explain why the FrameRgn is not working, and maybe provide some sample code here
that will make it work.

Answer 1

As shown in the CWnd::SetWindowRgn documentation:
After a successful call to SetWindowRgn, the operating system owns the
region specified by the region handle hRgn. The operating system does
not make a copy of the region, so do not make any further function
calls with this region handle, and do not close this region handle.
What this basically means is that you can't then go and use the region for another purpose, and you also can't "lose" the region. As it's a member variable, this last issue isn't a problem you need to worry about. But regarding the "do not use it" part, you will notice that the FrameRgn(...) call most likely returned zero, indicating the failure when trying to draw.
What you can do is to detach the region handle from the CRgn object and use that to set the window region, then you can recreate a new one as before:
m_rgnShape.CreateRoundRectRgn(...);
HGDIOBJ hRgn = m_rgnShape.Detach();
::SetWindowRgn(GetSafeHwnd(), (HRGN)hRgn, TRUE);
m_rgnShape.CreateRoundRectRgn(...);
For a better description, have a look at this article which covers Setting a Window Region, to make it look like a cat.
Edit: Your comment mentions that now, the framed region is effectively offset by an amount. The amount is likely to be the size of the border of your window.
When you call GetClientRect, it returns the size of the client area of the window - the part you can easily draw on, and the part that is "described" by the device context when you do CPaintDC dc(this); in your OnPaint() method.
The reason for the offset is that your window has a border, which you don't normally draw on (there are ways, but we'll ignore those for now). So the device context describes an area that's offset from your window.
The simplest solution to this in your case is likely to be to modify the dialog template to specify no borders. This will of course limit resizing the window, but as you've already set a region, I'm assuming resizing isn't an option either.