spy++ and WM_PAINT

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C#

Hello,

I have a 3rd party application with a chat box control, and I want to focus the parent window every time the chat box is updated.
I checked with spy++, and saw that the control receives P WM_PAINT every time a new line is added.

I tried hooking it myself (by hooking WH_GETMESSAGE) and writing to the log every time WM_PAINT is received, however the log showed nothing, even though spy++ showed activity.

I assume that the reason for that is that I checked if the message belongs to the control, and that it is indeed WM_PAINT before writing it the log, and the way I did it was by parsing the value of lParam:

CWPSTRUCT* cwps = (CWPSTRUCT*)lParam;
if ((cwps->hwnd != mWindowID) || (cwps->message != WM_PAINT))
  return CallNextHookEx(key, nCode, wParam, lParam);

It makes sense that the above condition will not pass, if for example lParam is null, which is EXACTLY what spy++ showed for every WM_PAINT message receives.

So my question is:
How does spy++ know that the specific control received a WM_PAINT message without the lParam?

Thanks in advance,
Elad