use of undefined type 'SelectMgr_Selection' ??? who can help me?

i define an own object:
void I3DGeom_Polyline::ComputeSelection(const Handle(SelectMgr_Selection)& aSelection,
const Standard_Integer aMode)
{
Handle( SelectMgr_EntityOwner ) own = new SelectMgr_EntityOwner(this, 0);
Handle( Select3D_SensitiveCurve ) SensCurve = new Select3D_SensitiveCurve( own, aCurve );
aSelection->Add( SensCurve );
}

but compiling tip:
: error C2027: use of undefined type 'SelectMgr_Selection'
e:\opencascade4.0\inc\handle_selectmgr_selection.hxx(33) : see declaration of 'SelectMgr_Selection'

who can help me?