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    <p>Well, start by asking yourself what could possibly go wrong with <code>class C&lt;T&gt; : T { }</code>. A huge number of things come immediately to mind:</p> <p>What if T is a struct? What if T is a sealed class type? What if T is an interface type? What if T is <code>C&lt;T&gt;</code>?! What if T is a class derived from <code>C&lt;T&gt;</code>? What if T is an abstract type with an abstract method? What if T has less accessibility than C ? What if T is System.ValueType? (Can you have a non-struct which inherits from System.ValueType?) What about System.Delegate, System.Enum, and so on? </p> <p>Those are the easy, obvious ones. The proposed feature opens up literally hundreds, if not thousands of more subtle questions about the interaction between the type and its base type, all of which would have to be carefully specified, implemented and tested. We'd undoubtedly miss some, and thereby cause breaking changes in the future, or saddle the runtime with implementation-defined behaviour. </p> <p>The costs would be enormous, so the benefit had better be enormous. I'm not seeing an enormous benefit here. </p>
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    1. POWhy cannot C# generics derive from one of the generic type parameters like they can in C++ templates?
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    1. USYannick Motton
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    1. CODo you find this feature less difficult to implement for C++ templates? Because it exists there and it is tremendously useful and exceptionally powerful. I can give one example when I would find this feature really useful, because that is related to my work at the moment. If you want to dynamically generate a type with Reflection.Emit, which base type is only known at run-time, then it would be hugely useful to have a statically compiled generic deriving from its type parameter, from which the dynamically emitted type could derive.
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    2. COIt is far less difficult to implement for C++ templates because C++ templates *recompile* the template on each construction of it. C# generics are compiled *once* and the same code is generated at runtime for ALL constructions if the type argument is a reference type. That's a fundamental difference; it means that C# generics must be correct for any *possible* construction, whereas C++ templates need only be correct for the finite number of *actual* constructions in the source code.
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    3. COGood point. But generic types are open types. One cannot have any instances of an open generic type. Hence its actual memory layout may remain unresolved until a generic type is actually instantiated. In fact, this is the case even today. A generic type aggregating an instance of the generic parameter type has unclear memory layout until instantiation. At the moment I am not convinced that it is impossible to come up with a set of constraints (new types of constraints will be needed) to make the discussed inheritance work.
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