C / C++ / MFC

   CWnd   hWnd;
// assign hWnd to current window handle (dialog app/dialog-based app window)

   static CWnd *pWnd;
   pWnd = FindWindow("RecvApp",0);
   pWnd->PostMessage(WM_CONNECT_APP, hWnd, 1);

   // in a header definition file
   CWnd   *pWnd_Caller;

// in MainFrame.cpp - main frame for "RecvApp"

   ON_MESSAGE(WM_CONNECT_APP, OnMakeConnection)

   LRESULT CMainFrame::OnMakeConnect(WPARAM wParam, LPARAM lParam)
   {
      // wParam is the handle to the caller's window/dialog-window

      if (lParam == 1)
         m_View->PostMessage(WM_CONNECT_APP, wParam, lParam);

   }

// in "RecvAppView.cpp" - view frame for "RecvApp"
   // somewhere in a message mapped function, I want to use wParam (hWnd) as
         wParam->PostMessage(WM_ADD_ENTITY, 0, 0);

(caller) PostMessage (to receiver)

(receiver) assign values, do its stuff...
(receiver) PostMessage (to caller - new values)