How to raise events with an Interface as argument

Hi,

I have to raise an event from my library and, one of the parameter is
an IDispatch derived interface:

HRESULT Fire_OnStreamStateChanged( IWaveStream* Sender, short
new_state )
{

  HRESULT hr = S_OK;
  T * pThis = static_cast<T *>(this);
  int cConnections = m_vec.GetSize();

  for (int iConnection = 0; iConnection < cConnections; iConnection++)
  {
    pThis->Lock();
    CComPtr<IUnknown> punkConnection = m_vec.GetAt(iConnection);
    pThis->Unlock();

    CComQIPtr< IDispatch, &IID_IDispatch > pDispatch
( punkConnection );

    if (pDispatch != NULL)
    {
      CComVariant avarParams[2];

      ....?????
      ....?????

      VariantInit(&avarParams[0]);
      avarParams[0].vt = ...???;
      avarParams[1] = new_state;

      DISPPARAMS params = { avarParams, NULL, 2, 0 };
      hr = pDispatch->Invoke(1, IID_NULL, LOCALE_USER_DEFAULT,
DISPATCH_METHOD, &params, NULL, NULL, NULL);
    }
  }
  return hr;
}

How I have to pack the WaveStream* Sender object?

Thanks for the suggestions!
Daniele.