>>805
それdynamic_cast関係なくダメだろ。
IClassB* pIClassB;
HRESULT hr = CoCreateInstance(
    CLSID_ClassB, 0, CLSCTX_INPROC_SERVER,
    IID_IClassB, (void **)pIClassB);
CClassB* pCClassB = dynamic_cast<CClassB*>(pIClassB);