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Window Stations and Desktops 5

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Window Stations and Desktops 6

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LsaLogonUser + CreateProcessAsUser - 0xc0000142

The method described in the previous coding blog entry works great if you are using the window visibility state to control painting, since you're using the paint system itself to do the heavy lifting for you.

To obtain this information outside of the paint loop, use GetDC and GetClipBox. The HDC that comes out of GetDC is clipped to the visible region, and then you can use GetClipBox to extract information out of it.

Start with our scratch program and add these lines:

void CALLBACK
PollTimer(HWND hwnd, UINT uMsg, UINT_PTR idTimer, DWORD dwTime)
{
    HDC hdc = GetDC(hwnd);
    if (hdc) {
        RECT rcClip, rcClient;
        LPCTSTR pszMsg;
        switch (GetClipBox(hdc, &rcClip)) {
        case NULLREGION:
            pszMsg = TEXT("completely covered"); break;
        case SIMPLEREGION:
            GetClientRect(hwnd, &rcClient);
            if (EqualRect(&rcClient, &rcClip)) {
                pszMsg = TEXT("completely uncovered");
            } else {
                pszMsg = TEXT("partially covered");
            }
            break;
        case COMPLEXREGION:
            pszMsg = TEXT("partially covered"); break;
        default:
            pszMsg = TEXT("Error"); break;
        }
        // If we wanted, we could also use RectVisible
        // or PtVisible - or go totally overboard by
        // using GetClipRgn
        ReleaseDC(hwnd, hdc);

        SetWindowText(hwnd, pszMsg);
    }
}

BOOL
OnCreate(HWND hwnd, LPCREATESTRUCT lpcs)
{
    SetTimer(hwnd, 1, 1000, PollTimer);
    return TRUE;
}

Once a second, the window title will update with the current visibility of the client rectangle.

Polling is more expensive than letting the paint system do the work for you, so do try to use the painting method first.