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POWhy restricting return type for method and property inheritance so that descendants are not allowed?
 

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    <p>Given the following hypothetical situation:</p> <pre><code>class ClassParent { } interface IClassProvider { ClassParent Get(); } </code></pre> <p>Why is this then illegal IClassProvider implementation:</p> <pre><code>class ClassChild : ClassParent, IClassProvider { ClassChild Get() { return this; } } </code></pre> <p>It also doesn't work for inheriting properties and implementing base class instead of an interface like in the example.</p> <p>ClassChild <em>is</em> ClassParent. Why does it not compile then? The compiler is clearly aware of a compile-time type of a class, that's why the following works:</p> <pre><code>void DoSomething(object o) { ... }; void DoSomething(ConcreteClass c) { ... }; DoSomething(new ConcreteClass()); //Calls the second overload of the method because of static type resolving </code></pre> <p>In layered scenarios this forces me to have a bunch of proxy-methods and needlessly clutter my code when the underlying situation is clear. Frankly I can't think of any problems or ambiguities if that was supported. I would be happy with compile-time (static) resolution, like it works with overloads in the second example.</p> <p>Edit: I know that the compiler expects the following:</p> <pre><code>class ClassChild : ClassParent, IClassProvider { ClassParent Get() { return this; } } </code></pre> <p>and I know that it would work, I'm asking for an explanation why is it not supported or a scenario in which it would cause either a) problems or b)ambiguities worse than with overloads, which are supported.</p> <p>Edit 2: This is as it seems another duplicate of a same old question as answered by apparently MS himself in <a href="https://stackoverflow.com/questions/1319663/why-c-doesnt-allow-inheritance-of-return-type-when-implementing-an-interface">this SO question</a>. I'm marking @Euphoric 's post as answer because he provided a name for the feature which helped finding the "solution".</p>
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