Why is NMHDR.code always 0 on x64?

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C#

Windows

Win7

ListView

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

The class is to implement a virtual ListView.

protected override void 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;

                     // ....

public struct NMHDR
    {
        /// <summary>hwndFrom</summary>
        public IntPtr hwndFrom;
        /// <summary>idFrom</summary>
        public int idFrom;
        /// <summary>code</summary>
        public int code;
    }

/// <summary>EXTERNALSYM LVN_FIRST</summary>
        public const int LVN_FIRST                = (0-100);    //listview

/// <summary>EXTERNALSYM LVN_ODCACHEHINT</summary>
        public const int LVN_ODCACHEHINT            = (LVN_FIRST - 13);
        /// <summary>EXTERNALSYM LVN_GETDISPINFO</summary>
        public const int LVN_GETDISPINFO            = (LVN_FIRST - 77);
        /// <summary>EXTERNALSYM LVN_ODFINDITEM</summary>
        public const int 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 the

nm.code

is 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?