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    <p>The point is that the interface represents a <em>contract</em>. A set of public methods any implementing class has to have. Technically, the interface only governs syntax, i.e. what methods are there, what arguments they get and what they return. Usually they encapsulate semantics as well, although that only by documentation.</p> <p>You can then have different implementations of an interface and swap them out at will. In your example, since every pizza instance is an <code>IPizza</code> you can use <code>IPizza</code> wherever you handle an instance of an unknown pizza type. Any instance whose type inherits from <code>IPizza</code> is guaranteed to be orderable, as it has an <code>Order()</code> method.</p> <p>Python is not statically-typed, therefore types are kept and looked up at runtime. So you can try calling an <code>Order()</code> method on any object. The runtime is happy as long as the object has such a method and probably just shrugs and says »Meh.« if it doesn't. Not so in C#. The compiler is responsible for making the correct calls and if it just has some random <code>object</code> the compiler doesn't know yet whether the instance during runtime will have that method. From the compiler's point of view it's invalid since it cannot verify it. (You can do such things with reflection or the <code>dynamic</code> keyword, but that's going a bit far right now, I guess.)</p> <p>Also note that an interface in the usual sense does not necessarily have to be a C# <code>interface</code>, it could be an abstract class as well or even a normal class (which can come in handy if all subclasses need to share some common code – in most cases, however, <code>interface</code> suffices).</p>
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    1. CO+1, although I wouldn't say that the interface (in sense of a contract) can be an abstract or a normal class.
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    2. COI would add that you can't expect to understand interfaces in a couple of minutes. I think it is not reasonable to understand interfaces if you don't have years of object oriented programming experience. You may add some links to books. I would suggest: [Dependency Injection in .NET](http://www.manning.com/seemann/) which is really how deep the rabit hole goes, not just a gentle introduction.
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    3. COAh, there's the problem of me having no clue of DI. But I guess the main problem of the asker was why they are needed when in Python it all works without. That's what the largest paragraph of my answer tried to provide. I don't think it's necessary to dig in every pattern and practice that uses interfaces here.
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