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    <p>Most of the time, you probably don't care. But here are some instances that I can think of where you would. There are several, and it does vary a little bit by language. Some languages don't mind as much as others.</p> <p>In the case of inversion of control (where someone PASSES you a parameter) you probably don't care if it's an interface or an object as far as calling its methods etc. But when dealing with types, it definitely can make a difference.</p> <ul> <li><p>In managed languages such as .NET languages, interfaces can usually only inherit one interface, whereas a class can inherit one class but implement many interfaces. The order of classes vs interfaces may also matter in a class or interface declaration. So you need to know which is which when defining a new class or interface.</p></li> <li><p>In Delphi / VCL, interfaces are reference counted and automatically collected, whereas classes must be explicitly freed, so lifecyle management on the whole is affected, not just the creation.</p></li> <li><p>Interfaces may not be viable sources for class references.</p></li> <li><p>Interfaces can be cast to compatible interfaces, but in many languages, they cannot be cast to compatible classes. Classes can be cast to either.</p></li> <li><p>Interfaces may be passed to parameters of type IID, or IUnknown, whereas classes cannot (without a cast and a supporting interface).</p></li> <li><p>An interface's implementation is unknown. Its input and output are defined, but the implementation which creates the output is abstracted. In general, ones attitude may be that when working with a class, one may know how the class works. But when working with an interface, no such assumption should be made. In a perfect world, it might make no difference. But in reality, this most certainly can have affect your design.</p></li> </ul>
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    1. POWhy should you ever have to care whether an object reference is an interface or a class?
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    1. USPhil Gilmore
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    1. CO1) In c#, interfaces can definitely inherit multiple interfaces. I'd love to hear the rationale behind such a decision however, any language in particular you had in mind? I'm not sure I understand the note about ordering, if you could elaborate on that I'd be much obliged!
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      1. USMarcus Stade
    2. CO2) Very interesting, I'll have to read up on that. I have no Delphi experience at all so obviously I can't comment further than that.
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    3. CO3) Not sure what you mean by this.
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