Debugging with Message Boxes

When debugging MFC applictaions, AfxMessageBox serves as a simple output mechanism for inspecting variable values at runtime.

CString debugOutput;
debugOutput.Format(_T("Current index value: %d"), index);
AfxMessageBox(debugOutput);

Controlling Window Resize Behavior

Dialog-Based Applications

Adjust the window border property in the resource editor. Select a non-resizable border style such as None (no border) or Thin instead of Resizing.

Single Document Interface Applications

Override the PreCreateWindow method in CMainFrame to modify window creation parameters:

BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs)
{
    if (!CFrameWnd::PreCreateWindow(cs))
        return FALSE;
    
    // Remove thick frame to disable resizing
    cs.style &= ~WS_THICKFRAME;
    
    // Remove maximize box
    cs.style &= ~WS_MAXIMIZEBOX;
    
    // Optionally remove minimize box
    cs.style &= ~WS_MINIMIZEBOX;
    
    return TRUE;
}

Capturing and Saving Window Content

Export the current window client area as an image file with format selection:

void CMyView::OnSaveSnapshot()
{
    CClientDC viewDC(this);
    CRect clientRect;
    GetClientRect(&clientRect);
    
    int width = clientRect.Width();
    int height = clientRect.Height();
    
    // Create a bitmap compatible with the display
    HBITMAP hBitmap = CreateCompatibleBitmap(viewDC, width, height);
    HDC hMemDC = CreateCompatibleDC(viewDC);
    HBITMAP hOldBitmap = (HBITMAP)SelectObject(hMemDC, hBitmap);
    
    // Copy pixels from screen to memory device context
    BitBlt(hMemDC, 0, 0, width, height, viewDC, 0, 0, SRCCOPY);
    
    CImage snapshot;
    snapshot.Attach(hBitmap);
    
    CString filter = _T("BMP Files(*.bmp)|*.bmp|JPEG Files(*.jpg)|*.jpg|PNG Files(*.png)|*.png|All Files(*.*)|*.*||");
    CFileDialog dialog(FALSE, _T("bmp"), _T("capture.bmp"), OFN_HIDEREADONLY | OFN_OVERWRITEPROMPT, filter);
    
    if (dialog.DoModal() != IDOK)
    {
        SelectObject(hMemDC, hOldBitmap);
        DeleteDC(hMemDC);
        return;
    }
    
    CString filePath = dialog.GetPathName();
    
    // Append appropriate extension if user omitted it
    if (dialog.m_ofn.nFileExtension == 0)
    {
        int filterIndex = dialog.m_ofn.nFilterIndex;
        CString ext;
        switch (filterIndex)
        {
        case 1: ext = _T("bmp"); break;
        case 2: ext = _T("jpg"); break;
        case 3: ext = _T("png"); break;
        default: ext = _T("bmp"); break;
        }
        filePath += _T(".") + ext;
    }
    
    HRESULT result = snapshot.Save(filePath);
    if (FAILED(result))
    {
        MessageBox(_T("Failed to save image."));
    }
    
    snapshot.Detach();
    SelectObject(hMemDC, hOldBitmap);
    DeleteDC(hMemDC);
}

The technique uses BitBlt to transfre the client area to a memory device context, then leverages CImage::Save to write the bitmap in the user's chosen format (BMP, JPEG, PNG, or others).