6.10 Supporting multiple interfaces

The Haskell COM component design detailed in the last couple of section can easily be extended to support multiple interface per component. For example, supposing the IntRef component also provide a method for drawing its current value via the IDraw interface:

[ object,
  uuid(C1DF9B10-BDDB-11d1-99CC-006097B7314A),
  pointer_default(unique)
]
interface IIntRef {
  HRESULT set([in] int x);
  HRESULT get([out] int* val);
}
[ object,
  uuid(C1DF9B12-BDDB-11d1-99CC-006097B7314A),
  pointer_default(unique)
]
interface IDraw {
  HRESULT Draw([in] HDC hdc);
}

[uuid(116E95C0-075E-11d2-A4E9-00A0C91F393A)]
coclass IntRef {
  [default]interface IIntRef;
  interface IDraw;
}

To accommodate this extra interface, the following changes will have to be made:

• The user-supplied implementation module for IntRef will have to additionally export an action named draw.
• Since state is represented per-component, and not per-interface, the State type may also have to be changed.
• The implementation of the supporting stubs have been designed to with multiple interfaces in mind, so the addition of a second interface is trivial to generate code for. ToDo:expand a bit.