Guidelines for COM Interoperability from .NET (Update 1)



Sometime back I posted
Guidelines for COM Interoperability from .NET where I
enumerated several practices to follow when exposing types from your managed
assemblies to COM. This update is to provide some additional information with
the release of the Microsoft .NET Framework 2.0 as well as correct one statement
I made with regard to inheritting interfaces when those interfaces are exposed
either as automation interfaces (inheriting from IDispatch) or dual interfaces
(inheriting both from IUnknown and IDispatch).

Previously I mentioned that you should declare class interfaces explicitly
and implement them as the first interface on your class attributed with the
ClassInterfaceAttribute with the value of ClassInterfaceType.None. In the .NET
Framework 2.0 a new attribute was added to explicitly declare which interface is
the default interface for the class since in some cases the provided compilers
will generate interfaces in a different order and third-party compilers may not
even honor the order in the first place. The ComDefaultInterfaceAttribute class
should be declared for your implementation class and you should specify the type
of the class interface. The class must still implement this interface. This is
also necessary since the interface will always inherit from either IUnknown or
IDispatch depending on the value you use for the InterfaceTypeAttribute
attributed on the interface. See

Inheritance, Aggregation, and Containment
in the .NET Framework SDK for more

I also mentioned that you should implement new versions of interfaces that
inherit from older interfaces. When exposing your interfaces with
ComInterfaceType.InterfaceIsIDispatch or ComInterfaceType.InterfaceIsDual you
should actually re-declare all previous members since many automation clients
like Windows Script can only work with a single default dispatch interface. You
should still implement the older versions of the interfaces but as explicit
interface implementations

Also note that there is a new namespace that you should use if you can base
your development and deployment with .NET 2.0 as the minimal requirement. The

System.Runtime.InteropServices.ComTypes namespace
declares many
interfaces commonly implemented by COM servers and used by COM clients. The
interfaces in System.Runtime.InteropServices that begin with UCOMI* are obsolete
and may be removed in future versions as well.

Heath Stewart

Senior Software Engineer

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