passing reference to pointf from vb to c++ both in vb.net

I have a reference to a pointf. the vb call is:

dim pta as pointf -- the declaration of pta

passref(pta) --- calling vc++

it is declared in vb as :
Declare Sub passref Lib "C:\vcnet\mfdm\debug\mfdm.dll" (
byref pta As PointF)

in vc++ it is declared as:

void FAR PASCAL passref (POINTF&pta)
{

pta.x=3 --- gives exception

}

upon entering the pass ref routine pta is unrecognized and seems to point to a memory location 0x00004


maybe I can't pass an object even though built in as pointf is to vc++

there is of course a workaround but not as nice as it could be.