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    <p>(Note whilst I'll use your example I'll try to concentrate on the concept rather then tell you what's wrong with your specific example).</p> <p><strong>Black Box Inheritance?</strong></p> <p>What you're crashing into is the myth of "Black box inheritance". Its often not actually possible to separate completely implementations that allow inheritance from implementations that use that inheritance. I know this flys in the face of how inheritance is often taught but there it is.</p> <p>To take your example, its quite reasonable for you to want the consumers of the collection contract to see a Count which matches the number items they can get out of your collection. Its also quite reasonable for code in the inherited base class to access its Count property and get what it expects. Something has to give.</p> <p><strong>Who is Responsible?</strong></p> <p>Answer: The base class. To achieve both the goals above the base class needs to handle things differently. Why is this the reponsibility of the base class? Because it allows itself to be inherited from and allowed the member implementation to be overriden. Now it may be in some languages that facilitate an OO design that you aren't given a choice. However that just makes this problem harder to deal with but it still needs be dealt with.</p> <p>In the example, the base collection class should have its own internal means of determining its actual count in the knowledge that a sub-class may override the existing implementation of Count. Its own implementation of the public and overridable Count property should not impact on the internal operation of the base class but just be a means to acheive the external contract it is implementing.</p> <p>Of course this means the implementation of the base class isn't as crisp and clean as we would like. That's what I mean by the black box inheritance being a myth, there is some implementation cost just to allow inheritance.</p> <p><strong>The Bottom Line...</strong></p> <p>is an inheritable class needs to be coded defensively so that it doesn't rely on assumed operation of overridable members. OR it needs to be very clear in some form of documentation exactly what behaviour is expected from overriden implementations of members (this is common in classes that define abstract members).</p>
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