Why CS_OWNDC doesnt work?

Code:

HDC hDC;
	HBITMAP hbitmap;

	// create bitmap brush	
    hbitmap = LoadBitmap(hinst,MAKEINTRESOURCE(IDB_BACKGROUND));
	hbrush = CreatePatternBrush(hbitmap);
	DeleteBitmap(hbitmap);
	hDC = GetDC(hwnd);
	SelectBrush(hDC,hbrush);
	ReleaseDC(hwnd,hDC);
	SelectBrush(hDC,GetStockBrush(BLACK_BRUSH));

Hi, I have the above code right after i created a certain window. However, the window was not painted with the bitmap brush i created. Can someone explan why? I thought the new brush will takes effect for the rest of the program life if I use the style CS_OWNDC.

If I set the hbrBackground field to the bitmap brush during the window class registration, the window is painted properly. I find this very strange.

CS_OWNDC Just gives each window its own device context.

I have only made one window. It has its own DC and it is suppose to retain any changes in its DC attributes right? So if I change the brush after creating the window, it should be able to paint the window with the new bitmap brush or at least that's what I know.

Ah ha! It looks like you need to select the brush to the HDC before deleting the original bitmap.

I am quite sure i can delete the bitmap before i select the brush. Anyway i tried to select it in without deleting the bitmap. It still didnt work.

Code:

	ReleaseDC(hwnd,hDC);
	SelectBrush(hDC,GetStockBrush(BLACK_BRUSH));

Why do you select a brush after you've released the DC?

The problem comes from there. Since the DC is an OWNDC, ReleaseDC has no effect on it. Thus the handle is still valid after the ReleaseDC call (though it's poor coding to still use it), and the SelectBrush with the BLACK_BRUSH has an effect. In other words, you're resetting the brush back to black.

But to set the brush in the WNDCLASS would be a better method anyway. Especially since you don't rely on CS_OWNDC that way.

I didn't notice that he did that. Yeah that would be a major problem. There are some memory issues to consider when using CS_OWNDC depending on what you are doing too. CS_OWNDC is great for a window that serves as a canvas (i.e. a paint program), but in an app that is heavy on controls it tends to waste memory.

ack...sorry i was doing cut and paste to replicate the problem and the SelectBrush() wasn't there at first.

Here is the actual problem:

Code:

hDC = GetDC(hwnd); 
		

	HBITMAP hbitmap;

	// create bitmap brush	
    hbitmap = LoadBitmap(hinst,MAKEINTRESOURCE(IDB_BACKGROUND));
	hbrush = CreatePatternBrush(hbitmap);
	DeleteBitmap(hbitmap);

	SelectBrush(hDC,hbrush);
	ReleaseDC(hwnd,hDC);

My background brush was initially (COLOR_WINDOW+1) for the hbrBackground field.

list of things i tried AFTER CREATING THE WINDOW and didnt work
:
1. Delete bitmap after selecting brush

Code:

	hDC = GetDC(hwnd); 
		

	HBITMAP hbitmap;

	// create bitmap brush	
    hbitmap = LoadBitmap(hinst,MAKEINTRESOURCE(IDB_BACKGROUND));
	hbrush = CreatePatternBrush(hbitmap);
	SelectBrush(hDC,hbrush);
	DeleteBitmap(hbitmap);
	ReleaseDC(hwnd,hDC);

2. select black brush into hDC, the background remains white instead of black

Code:

	hDC = GetDC(hwnd); 
	
	HBITMAP hbitmap;

	// create bitmap brush	
    hbitmap = LoadBitmap(hinst,MAKEINTRESOURCE(IDB_BACKGROUND));
	hbrush = CreatePatternBrush(hbitmap);
	SelectBrush(hDC,hbrush);
	DeleteBitmap(hbitmap);
        SelectBrush(hDC,GetStockBrush(BLACK_BRUSH));
	ReleaseDC(hwnd,hDC);

BTW, I am using private DCs because i will be making two windows base on the same window class. But their background will be of different bitmap brush.

It might be that, contrary to the documentation, ReleaseDC resets the DC or GetDC does.

Removed all the ReleaseDC()s and EndPaint()...it still didnt work.
The strangest thing is that i checked the brush in the hDC using GetCurrentObject() and OBJ_BRUSH during every WM_PAINT msg. It has the right bitmap brush, but it just doesnt paint the window with that brush.

I can just fill the brush field with the bitmap brush before creating the window and make it work. But i really want to know what is wrong with the other method.

Perhaps the DC used to draw the background is different?
Catch the WM_ERASEBKGND message. wParam is a device context, you could check that.
Besides, the way I read the WM_ERASEBKGND docs, DefWindowProc ignores the currently selected brush and automatically selects the one from the WNDCLASS. So, either you change it in the WNDCLASS or you intercept the message and erase the background yourself.

I managed to solve the problem by adding PatBlt while handling WM_PAINT msg.

Code:

void mainFrame::mainWindow_onPaint(HWND hwnd)
{
	PAINTSTRUCT ps;

	HDC hDC = BeginPaint(hwnd,&ps);
	PatBlt(hDC,0,0,512,512,PATCOPY);
	EndPaint(hwnd,&ps);
	
}

However, if i fill the hbrBackground field with the bitmap brush and do a PatBlt. I get a white screen again. Strange!

Hmm, i have encountered yet another problem, so i'm gonna post it here.

Code:

	// setup common controls
	icc.dwSize = sizeof(INITCOMMONCONTROLSEX);
	icc.dwICC = 0x00004000;//ICC_STANDARD_CLASSES
	InitCommonControlsEx(&icc);

Visual Studio .NET 2003 doesnt recognize ICC_STANDARD_CLASSES and complains it's a undeclared identifier. It is definitely found in <commctrl.h> and i have included that header file.

My edit control works without calling InitCommonControlsEx() or if i hardcode the value of dwICC. But Microsoft says i must call it to initialize controls and i dont want to expect any strange problem later.

Can someone help? Thanks alot!