OnNotify Not Called

I'm sending a WM_NOTIFY message to another class, which has a message map set up to handle the notification. However, the function I've set up in the message map is never called. The code is below:

TooltipBase.h

Code:

---


    //! @brief Information shown in a tooltip
    struct TooltipData
    {
        CString strHeader;
        CString strContents;
        CString strFooter;
    };

    //! @brief ON_NOTIFY structure
    struct TooltipNotification
    {
        NMHDR hdr;
        TooltipData* pTooltipData;
    };

---

TooltipBase.cpp

Code:

---


    // Code to send the message.
    TooltipNotification ttn;
    ttn.hdr.hwndFrom = m_hParentHWnd;
    ttn.hdr.idFrom  = GetDlgCtrlID();
    ttn.hdr.code    = UDM_NEED_TOOLTIP_TEXT;
    ttn.pTooltipData = &m_TooltipData;   
    ::SendMessage( m_hParentHWnd, WM_NOTIFY, ( WPARAM ) m_hParentHWnd, ( LPARAM ) &ttn );

---

Canvas2.cpp - Just a CWnd class that shapes are drawn on.

Code:

---


    ON_NOTIFY          ( UDM_NEED_TOOLTIP_TEXT, NULL, OnNeedTooltipText )

    ....

    void Canvas2::OnNeedTooltipText( NMHDR* pNotifyStruct, LRESULT * result )
    {
        TRACE( _T( "The tooltip needs some text." ) );
    }

---

The message appears to be sent ok, as the program continues, but I can't be sure what is happening to the message (nor if I'm right that it is being sent).

I never sent WM_NOTIFY myself, therefore can only guess:
why ddo you use different values of *control ID* (WPARAM) in the SendMessage call (where it is m_hParentHWnd) and in the message handler (where it is NULL)?