Background combobox on gradient parent window? - c++

Question

I have window with gradient background. Combobox have own background brush.
How can I remove white corner in combobox? How can I change brush or another way.
On picture white corner marked by red frame.
I create combobox as:
DWORD dwStyle = WS_CHILD | CBS_DROPDOWNLIST;
if (m_bVisible) dwStyle |= WS_VISIBLE;
m_hWnd = CreateWindow(WC_COMBOBOX, NULL, dwStyle,
m_posX, m_posY, m_width, m_height, m_hParent, (HMENU)m_id, m_hInstance, NULL);
I tried changing background brush with message WM_CTLCOLOREDIT, but no effect:
case WM_CTLCOLOREDIT:
if ((HWND)lParam == m_hSrcListBox)
{
return (LRESULT)m_hBrush;
}
break;
=== SOLVED. WORK VERSION ===
First way.
In parent WndProc:
case WM_CTLCOLORSTATIC:
if ((HWND)lParam == m_hSrcListBox)
{
return (LRESULT)m_pSrcListBox->GetHbrush();
}
break;
In my class:
//
// CListBox::GetHbrush().
//
// Get brush.
//
HBRUSH CListBox::GetHbrush()
{
if (!m_hBrush)
{
m_hBrush = CreateTransparentBackgroundBrush(m_hParent, m_hWnd);
}
return m_hBrush;
}
Create transparent background:
//
// CListBox::CreateTransparentBackgroundBrush().
//
// Create transparent background for element.
//
HBRUSH CListBox::CreateTransparentBackgroundBrush(HWND parent, HWND client)
{
RECT rct;
POINT p1;
POINT p2;
GetWindowRect(client, &rct);
p1.x = rct.left;
p1.y = rct.top;
ScreenToClient(parent, &p1);
p2.x = rct.right;
p2.y = rct.bottom;
ScreenToClient(parent, &p2);
HDC hdcParent = GetDC(parent);
HDC hdcClient = GetDC(client);
HDC hdcmem = CreateCompatibleDC(hdcClient);
HBITMAP hbitmap = CreateCompatibleBitmap(hdcClient, p2.x - p1.x, p2.y - p1.y);
SelectObject(hdcmem, hbitmap);
BitBlt(hdcmem, 0, 0, p2.x - p1.x, p2.y - p1.y, hdcParent, p1.x, p1.y, SRCCOPY);
HBRUSH pattern = CreatePatternBrush(hbitmap);
DeleteDC(hdcmem);
DeleteObject(hbitmap);
ReleaseDC(client, hdcClient);
ReleaseDC(parent, hdcParent);
return pattern;
}
Second way.
In parent WndProc draw background in WM_ERASEBKGND message, then the corners will not.
case WM_ERASEBKGND:
m_hdc = (HDC)wParam;
// draw background.
return TRUE;
break;
The result of both methods:

Answer 1

For dialog boxes, handle WM_CTLCOLORDLG and return a background brush for the combobox
If you are displaying this combobox in a dialog, the trick is actually to handle the WM_CTLCOLORDLG message in your dialog's window procedure. In response to this message, you return a handle to a brush that the dialog box will use to paint its background.
case WM_CTLCOLORDLG:
{
// NOTE: This code is wrong because it creates a new brush object each time it processes
// the message, which it promptly leaks. It is merely for demonstration purposes.
// Normally, you would create the brush once, in response to WM_INITDIALOG,
// cache it away, and return that same cached handle each time, finally destroying
// the brush in response to WM_NCDESTROY.
HBRUSH hBrush = CreateSolidBrush(RGB(255, 120, 0));
return reinterpret_cast<INT_PTR>(hBrush);
}
                    
This is the standard, documented way of changing the background color of a dialog box, and it also solves the problem with the combobox. Apparently, for whatever reason, combobox controls also use this brush to paint their background. I suppose they send a WM_CTLCOLORDLG message to their parent when they are painting themselves.
Of course, this limits you to the graphics capabilities of a GDI brush. You can draw any system or solid color that you want, or even use a hatch or pattern/bitmap brush, but there is no simple way of creating a gradient brush. (GDI+ has one, but not GDI.) Normally it wouldn't matter—you'd just call the GradientFill function in your WM_PAINT (or even WM_ERASEBKGND) message handler. That works fine for the dialog's background, but the combobox still draws its background with the brush returned by WM_CTLCOLORDLG, so it still has those 4 dots on its corners drawn in COLOR_3DFACE (which is the brush that the default dialog procedure returns).
                    
Returning a null brush (NULL_BRUSH/HOLLOW_BRUSH) from WM_CTLCOLORDLG doesn't work, either. It changes the appearance slightly, such that the upper-right and lower-left corner pixels are now filled with something that looks like COLOR_3DSKSHADOW, but they are still visibly filled with a color other than the actual background gradient.
                    
So if you really want it to look nice, you are left with only a single option: returning a handle to a GDI brush. And of course, it needs to be the same brush as is used to draw the dialog's background.
If you want a gradient fill, the only solution I can think of is using a pattern/bitmap brush, where the bitmap (DDB or DIB) is your gradient. Not great, but at least the days of Windows 9x limiting us to 8×8 patterns are long gone. Maybe someone more inventive than me can use this information to think of a better workaround?
For other windows, handle WM_CTLCOLORSTATIC and return a background brush for the combobox
All of that for a dialog box. But what about if you are displaying the combobox in a standard window (i.e., something other than a dialog box)? The WM_CTLCOLORDLG message is never sent in this case.
Instead, the combobox sends a WM_CTLCOLORSTATIC message to its parent window, and then uses the brush handle returned in response to that message to paint its background.
This is weird, I know. I only stumbled across it by conducting empirical tests, and I'm not sure quite sure what the rationale was. If I had to guess, I'd say that the CBS_DROPDOWNLIST style makes the combobox non-editable (i.e., it's not a true combobox because there is no Edit control), so instead of WM_CTLCOLOREDIT, it uses WM_CTLCOLORSTATIC. A disabled Edit box sends WM_CTLCOLORSTATIC, too, and so does a disabled combobox with the "normal" CBS_SIMPLE and CBS_DROPDOWN styles.
Weirder still, this only happens when the Aero theme is enabled (Vista and 7). It doesn't happen on Windows 10, or with the Luna theme (Visual Styles under XP), or with the Classic theme. (I didn't test on Windows 8 or 8.1.) Not that it matters, I suppose, since all of those other themes draw a simple rectangular combobox, leaving no corner pixels for the background to show through.
Whatever the logic, the solution remains to handle the WM_CTLCOLORSTATIC message and return the brush you wish the combobox to use to paint its background.
The same considerations apply here as those discussed above for the dialog box. If your window uses a solid-color background or a system color, you are home-free. Simply return a handle to the same brush that you set as the window class's background brush. If you want to use a gradient, you'll need to figure out a way to represent that gradient in the form of a GDI brush.
WNDCLASSEX wcex;
wcex.cbSize = sizeof(WNDCLASSEX);
wcex.style = CS_HREDRAW | CS_VREDRAW;
wcex.lpfnWndProc = WndProc;
wcex.cbClsExtra = 0;
wcex.cbWndExtra = 0;
wcex.hInstance = hInstance;
wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDR_APPLICATION));
wcex.hIconSm = LoadIcon(hInstance, MAKEINTRESOURCE(IDR_APPLICATION_SMALL));
wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
wcex.hbrBackground = reinterpret_cast<HBRUSH>(COLOR_3DDKSHADOW + 1); // background brush
wcex.lpszMenuName = NULL;
wcex.lpszClassName = TEXT("My Colored Window Class");
RegisterClassEx(&wcex);
case WM_CTLCOLORSTATIC:
{
// NOTE: No leak here because we're using a system brush in this example.
return reinterpret_cast<LRESULT>(GetSysColorBrush(COLOR_3DDKSHADOW)); // background brush
}
                    

Answer 2

How can i remove white corner in combobox?
I don't know if there is a more official way to get rid of it, but one option would be to create a Region with rounded corners using CreateRoundRectRgn(), and then apply it to the ComboBox using SetWindowRgn(). That will mask off the corners.