cast from Iunknown to another type

hi , 

i need to cast a variable from Iunknown type to another type(SpaceClaim::Api::V10::Session^)

i m working with c++/cli  and i can't find any solution to fix that prb 

here is the code :

gcroot<SpaceClaim::Api::V10::Session^> m_piApplication;

STDMETHODIMP CEMWCADLinkCtrl::SetEMWCADLinkApplication(IUnknown* pCADApplication)
{

if ( pCADApplication == NULL )
return S_FALSE;


m_piApplication = (SpaceClaim::Api::V10::Session^)pCADApplication; 


return S_OK;

}

plz if there is some one how can help ill be very gratful 

thks,