Win32 HDC issue

someone 20

I have a problem with drawing HDC outside WM_PAINT message, I draw in function with while(true) loop

void quadcycle() 
{
	while(true) 
	{
		hdc = GetDC(hwnd);
		RECT rc = {50,50,150,150};
		FillRect(hdc, &rc, (HBRUSH)CreateSolidBrush(RGB(255,0,std::time(NULL)*2 % 255 ))) );
		ReleaseDC(hwnd,hdc);
	}
}

To make the square change color with time, but instead of changing color I saw my square disappear :(

I checked all the places and realized that it is because of this loop, but I just slow down the loop to make it disappear a little later.

Please help, I am new to win32 development.

David Lowndes 2,545 Reputation points MVP

2024-11-06T21:53:08.3833333+00:00

You don't appear to be deleting the HBRUSH you create - use DeleteObject as noted in the CreateSolidBrush documentation.
Castorix31 85,806 Reputation points

2024-11-06T22:00:07.2333333+00:00

And you should never draw outside WM_PAINT (apart cases like WS_EX_NOREDIRECTIONBITMAP)
Darran Rowe 1,031 Reputation points

2024-11-06T23:41:38.19+00:00

@Castorix31

Unfortunately, Using the GetDC Function and the two assoicated pages show that drawing outside of WM_PAINT is supported. It also shows that there are scenarios where drawing outside of WM_PAINT is perfectly reasonable.

The general issue with drawing outside of WM_PAINT is that it is much easier to cause issues with the window. There are two obvious problems. The first is that the changes can be lost if the changes are not saved internally and WM_PAINT is sent to the window. The second is that a badly written function can block the main thread.

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Win32 HDC issue

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