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    <p>From <a href="http://blogs.msdn.com/b/csharpfaq/archive/2004/03/07/85562.aspx" rel="noreferrer">Chris Brumme</a>:</p> <blockquote> <p>There are a number of reasons we don't implement Multiple Implementation Inheritance directly. (As you know, we support Multiple Interface Inheritance).</p> </blockquote> <p>I think what Krzysztof Cwalina states in the quote is not the concept of interfaces itself, but Multiple interface inheritance as a method of multiple inheritance. </p> <blockquote> <p>There are several reasons we haven't provided a baked-in, verifiable, CLS-compliant version of multiple implementation inheritance:</p> <ol> <li><p>Different languages actually have different expectations for how MI works. For example, how conflicts are resolved and whether duplicate bases are merged or redundant. Before we can implement MI in the CLR, we have to do a survey of all the languages, figure out the common concepts, and decide how to express them in a language-neutral manner. We would also have to decide whether MI belongs in the CLS and what this would mean for languages that don't want this concept (presumably VB.NET, for example). Of course, that's the business we are in as a common language runtime, but we haven't got around to doing it for MI yet.</p></li> <li><p>The number of places where MI is truly appropriate is actually quite small. In many cases, multiple interface inheritance can get the job done instead. In other cases, you may be able to use encapsulation and delegation. If we were to add a slightly different construct, like mixins, would that actually be more powerful?</p></li> <li><p>Multiple implementation inheritance injects a lot of complexity into the implementation. This complexity impacts casting, layout, dispatch, field access, serialization, identity comparisons, verifiability, reflection, generics, and probably lots of other places.</p></li> </ol> </blockquote>
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    1. POHow did C#'s lack of multiple inheritance lead to the need for interfaces?
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    1. COEven more fundamental than those issues is that in a diamond scenario, intermediate-layer classes which derive from a common base must either share the same base-class instance or use separate base-class instances. Sharing the base-class instance will make it difficult or impossible for derived classes to enforce their invariants. Using separate instances will make identity-preserving upcasts and downcasts impossible (since the base-class instance associated with a reference will depend upon the sequences of casts performed on it). In other words, .NET fundamentally violates .NET axioms.
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