How can I change the background brush for a window class?

I need to change the background brush for a window, when a user enters dragging an OLE object.

My code is as follows.

SetClassLongA is actually called. hWnd, DragBrush are OK.

But the background of the window hWnd does not changed. What am I doing wrong?

Is it right to use SetClassLongA in order to change the window background?

// IDropTarget implementation
virtual HRESULT __stdcall DragEnter(IDataObject* pDataObject, DWORD grfKeyState, POINTL pt, DWORD* pdwEffect) override {
 
    std::cout << "               OLE -> DragEnter." << std::endl;
 
    *pdwEffect = DROPEFFECT_COPY;
 
    HBRUSH DragBrush = CreateHatchBrush(HS_CROSS, RGB(255, 0, 0));
 
    if (DragBrush == NULL) {
        std::cout << "Error creating dragging brush_2" << std::endl;
    }

    DWORD OldBrush = SetClassLongA(hWnd, GCLP_HBRBACKGROUND, (LONG) DragBrush);

    BOOL ok1 = InvalidateRect(hWnd, NULL, TRUE);
    BOOL ok2 = UpdateWindow(hWnd);
    // ok1=ok2=1   OK
 
 
    return S_OK;
}

You can see remarks, it says: Use the SetClassLong function with care. For example, it is possible to change the background color for a class by using SetClassLong, but this change does not immediately repaint all windows belonging to the class.

So you need to use GetClientRect and FillRect to redraw the client area.

I used Visual Studio's project sample. See, here is your purple window before redraw:

When we add GetClientRect and FillRect. It shows correct color as I made the read one:

The code in WM_PAINT message:

case WM_PAINT:
    {

        PAINTSTRUCT ps;
        HDC hdc = BeginPaint(hWnd, &ps);
        // TODO: Add any drawing code that uses hdc here...
        HBRUSH hBrush = CreateSolidBrush(RGB(220, 0, 0));
        ULONG_PTR res = SetClassLongPtr(hWnd, GCLP_HBRBACKGROUND, (LONG)hBrush);
        //DeleteObject(hBrush);

        RECT rect;
        GetClientRect(hWnd, &rect);
        int res1= FillRect(hdc, &rect, hBrush);
        EndPaint(hWnd, &ps);
    }