dual
[uuid, oleautomation, dual [, optional-attribute-list]]
interface interfacename
{ . . .};
Example
[uuid(. . .), oleautomation, dual]
interface
IHello : Idispatch
{
//diverse
and sundry properties and methods defined here
};
Remarks
The dual attribute
identifies an interface that exposes properties and methods through IDispatch
and directly through the VTBL. The interface must be compatible with OLE
Automation and be derived from IDispatch. The attribute is not allowed
on dispinterfaces.
The dual
attribute creates an interface that is both a Dispatch interface and a
Component Object Model (COM) interface. The first seven entries of the VTBL for
a dual interface are the seven members of IDispatch, and the remaining
entries are OLE COM entries for direct access to members of the dual interface.
All the parameters and return types specified for members of a dual interface
must be OLE Automation-compatible types.
All members
of a dual interface must pass an HRESULT as the function return value. Members,
such as property accessor functions, that need to return other values, such
as should specify the last parameter as
[out, retval],
indicating an output parameter that returns the value of the function. In
addition, members that need to support multiple locales should pass an lcid parameter.
A dual
interface provides for both the speed of direct VTBL binding and the flexibility
of IDispatch binding. For this reason, dual interfaces are recommended
whenever possible.
Note If your
application accesses object data by casting the this pointer within the
interface call, you should check the VTBL pointers in the object against your
own VTBL pointers to ensure that you are connected to the appropriate proxy.
Specifying dual
on an interface implies that the interface is compatible with OLE Automation,
and therefore causes both the TYPEFLAG_FDUAL and TYPEFLAG_FOLEAUTOMATION flags
to be set.
Flags
TYPEFLAG_FDUAL,
TYPEFLAG_FOLEAUTOMATION
See Also