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CoClasses and Class Factories

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A COM object is an instance of a CoClass, which is a class that implements one or more COM interfaces. The COM object provides the services as defined by its interfaces.

CoClasses are instantiated by a special type of object called a class factory. Whenever an object's services are requested by a client, a class factory creates an object instance for that particular client. Typically, if another client requests the object's services, the class factory creates another object instance to service the second client. (Clients can also bind to running COM objects that register themselves to support it.)

A CoClass must have a class factory and a class identifier (CLSID) so that it can be instantiated externally, that is, from another module. Using these unique identifiers for CoClasses means that they can be updated whenever new interfaces are implemented in their class. A new interface can modify or add methods without affecting older versions, which is a common problem when using DLLs.

Delphi wizards take care of assigning class identifiers and of implementing and instantiating class factories.

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