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general, I recommend using a type library definition, but this is not a cut-anddried issue There are a number of things to consider when deciding where to define your interfaces VB doesn't distinguish between a class created as an interface definition and a class created as a real class Although a class module used to define an interface generally contains nothing but empty procedures, VB must provide an implementation of that class nonetheless In other words, you end up with dead code in your project VB classes are all derived from IDispatch If you're communicating via interfaces, you're not using IDispatch Forcing VB to provide an IDispatch implementation that is never called is a waste of code and resources If you expose implemented interfaces to scripting languages, you need to support IDispatch Otherwise, it's just baggage If you implement an interface in a Public class, you must also set the Instancing property of the VB class whose primary interface you're implementing to PublicNotCreatable or higher VB should provide a Private Implements to keep project-specific interface implementations from being exposed in the project's type library, but it doesn't If you want to use Implements but don't want to allow external access to the interface, put the definition in a type library, and don't redistribute the external library As long as you don't use the interface across thread boundaries, you have effectively "privatized" your interface The type library VB generates has an unresolvable reference to your private type library, but that doesn't compromise the integrity of the library in any way If this problem really bothers you, see "Post-Build Type Library Modifications" in 15 for a discussion of removing the interface entry altogether Bug alert: In VB5, an unresolvable interface reference in a custom control causes the generation of an invalid OCA file In particular, any interfaces that follow the dangling one including the interface describing your events are also lost The only work-around in VB5 is to ship and register the private type libraries to the developers who use your control or modify the type library Or you can upgrade to VB6
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If you need to use an interface across process boundaries, the type library for that interface must be registered on the target machine, and that means deploying and registering the type library with your finished product If you use a PublicNotCreatable class in your project to define the interface, you don't need the external library Alternately, you can use the PowerVB Post-Build Type Library Modifier add-in to import an external type library into your executable's type library This allows you to ship a single binary
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Although VB classes inherit from provided implementations of the IDispatch interface (which in turn inherits from IUnknown), VB does not support inheritance in code that you write Inheritance is a common OOP design construct that allows a class to surface the implementation of an interface from another class The class that provides the implementation is known as a base class Inheritance also allows you to override specific functions in the base class's implementation Function overriding lets you provide a custom implementation of the interface on a function-by-function basis rather than being forced to provide an implementation for every function in the interface Some languages, such as C+ + , allow for multiple inheritance, which lets you derive implementation for multiple interfaces Other languages, such as Java, allow only for single inheritance VB doesn't support inheritance, so you have to be a little more creative to reuse an implementation Although it would be nice to have inheritance in VB for many situations, inheritance-based systems are not necessarily all they're cracked up to be To show you that the grass is not always greener, I'll take a minute to discuss some of the weaknesses of inheritance The first problem is known as a fragile base class When you use inheritance, you often get into a situation in which multiple classes inherit from a single base class Then you find that you need to change the base class, but a base class change modifies the behavior of other classes as well Your development manager won't let you destabilize the whole system by modifying the base class two weeks before the ship date, so you end up overriding many if not all of the base class's functions This introduces additional problems, especially if you have several base classes between your class and the base class that's
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