Re: posting messages inside a regular mfc dll

I'm trying to figure why code that look similar to this does not call
my message handler in my CWinApp derived class:

#define WM_MY_MSG (WM_USER + 200)

// header
class CMyApp : public CWinApp
{
public:
  ...
  void OnMessage(WPARAM,LPARAM);
  DECLARE_MESSAGE_MAP()

};

// cpp
BEGIN_MESSAGE_MAP(CMyApp, CWinApp)
  ON_THREAD_MESSAGE(WM_MY_MSG,OnMessage)
END_MESSAGE_MAP()

// exported function from regular mfc dll
#ifdef __cplusplus // If used by C++ code,
extern "C" { // we need to export the C interface
#endif

__declspec(dllexport) BOOL __cdecl ExportedFunction()
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  PostThreadMessage(AfxGetApp()->m_nThreadID, WM_MY_MSG, 0, 0);
  return TRUE;

}

#ifdef __cplusplus}

#endif

// user message handler
void CMyApp::OnMessage(WPARAM wParam,LPARAM lParam)
{
  AFX_MANAGE_STATE(AfxGetStaticModuleState());
  AfxMessageBox(_T("OnDestroyWindow()"));

}