Does DefWindowProc while processing WM_ERASEBKGND use update region?

Hi,
I process WM_ERASEBKGND message for WS_CHILD window:

Code:

---

  case WM_ERASEBKGND:
    GetUpdateRect(hWnd,&urect,FALSE);
    //...........

---

In urect i always get {0,0,0,0}. That's why I ask: does DefWindowProc while processing WM_ERASEBKGND use update region or erase entire client area?

Sorry, I was mistaken. In urect there is no always {0,0,0,0}. But my question still in force: Does DefWindowProc while processing WM_ERASEBKGND use update region or erase entire client area?

I answer my question myself(it was checked) :) - Yes. But when BeginPaint send WM_ERASEBKGND internally, the update rect is always {0,0,0,0}. Why?
I know that before returning BeginPaint validate the update rect and that's why it is always {0,0,0,0}. But how DefWindowProc or my own code in processing WM_ERASEBKGND message will know update region if GetUpdateRect always return {0,0,0,0}?