Designing a COM Object

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When designing the COM object, you need to decide what COM interfaces you want to implement. You can write a COM object to implement an interface that has already been defined, or you can define a new interface for your object to implement. In addition, you can have your object support more than one interface. For information about standard COM interfaces that you might want to support, see the MSDN documentation.

  • To create a COM object that implements an existing interface, use the COM Object wizard.
  • To create a COM object that implements a new interface that you define, use either the COM Object wizard or the Automation Object wizard. The COM object wizard can generate a new default interface that descends from IUnknown, and the Automation object gives your object a default interface that descends from IDispatch. No matter which wizard you use, you can always use the Type Library editor later to change the parent interface of the default interface that the wizard generates.

In addition to deciding what interfaces to support, you must decide whether the COM object is an in-process server, out-of-process server, or remote server. For in-process servers and for out-of-process and remote servers that use a type library, COM marshals the data for you. Otherwise, you must consider how to marshal the data to out-of-process servers.