RegisterEvent C++ DLL wrapping in C#

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C++

C#

i' ve a serious trouble with a C++ DLL for implement a callback event and use this in C#. Those are the two native function :

long FUNC_RegisterEventHandler(EventHandler handler, BYTE evmask, LONG param);

void EventHandlerFunc(WORD event_id, LPVOID data, LONG param);

My implementation:

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
    public delegate void CCC_EventHandler(string event_id, object data, int param);
    [DllImport(@"QSN.DLL")]
    public static extern int CCC_RegisterEventHandler(CCC_EventHandler handler, byte evmask, int param);

private void RegisterEvent() {
        QSN_Library.CCC_EventHandler delegato = FireEvento();
        IntPtr val = System.Runtime.InteropServices.Marshal.GetFunctionPointerForDelegate(delegato);
        //
        QSN_Library.CCC_RegisterEventHandler(delegato, 0xff, 0);
        System.Runtime.InteropServices.Marshal.GetDelegateForFunctionPointer(val, 
        typeof(QSN_Library.CCC_EventHandler)); 
    }

    private QSN_Library.CCC_EventHandler FireEvento() {
        return new QSN_Library.CCC_EventHandler((string a, 
        object b, int c) => { 
        // ....
        });
    }

Nothing to do... Any suggestion ?