Why is NMHDR.code always 0 for Virtual ListView messages on Win7 x64

I have the following C# code on Win7 64bit, Visual Studio 2008.

The class is to implement a virtual ListView.

        protectedoverridevoid WndProc(ref Message m)
        {
              bool bHandled =false;

             switch (m.Msg)
            {
                 case MessageId.OCM_NOTIFY: // Reflected messages
                    NMHDR nm = (NMHDR) m.GetLParam(typeof(NMHDR));

                    switch (nm.code)
                    {
                        case CommCtrl.LVN_GETDISPINFO:
                           //do something
                           break;
                        case CommCtrl.LVN_ODCACHEHINT:
                           //do something
                           break;
                        case CommCtrl.LVN_ODFINDITEM:
                           //do something
                           break;

                          // ....

    publicstruct NMHDR
    {
            ///<summary>hwndFrom</summary>
            publicIntPtr hwndFrom;
            ///<summary>idFrom</summary>
            publicint idFrom;
            ///<summary>code</summary>
            publicint code;
    }

//<summary>EXTERNALSYM LVN_FIRST</summary>
publicconstint LVN_FIRST                = (0-100);   //listview

///<summary>EXTERNALSYM LVN_ODCACHEHINT</summary>
publicconstint LVN_ODCACHEHINT            = (LVN_FIRST -13);
///<summary>EXTERNALSYM LVN_GETDISPINFO</summary>
publicconstint LVN_GETDISPINFO            = (LVN_FIRST -77);
///<summary>EXTERNALSYM LVN_ODFINDITEM</summary>
publicconstint LVN_ODFINDITEM             = (LVN_FIRST -79);                                               

                                                                                                                                                                                                               It works well when I compile it as x86 platform. The nm.code is sometimes -13, or -77 or -79. But it always 0 when I compile as x64. In this way, the following switch code will never be executed any more. Does anyone know why this happens?