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POWhy cannot C# generics derive from one of the generic type parameters like they can in C++ templates?
 

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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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    <p>Why cannot C# generics derive from one of the generic type parameters like they can in C++ templates? I mean I know it impossible because CLR does not support this, but why?</p> <p>I am aware of the profound differences between C++ templates and C# generics - the former are compile time entities and must be resolved during the compilation, while the latter are first class run-time entities.</p> <p>Still, I am failing to see <strong>the reason</strong> why CLR designers did not come up with a scheme which would ultimately enable a CLR generic type to derive from one of its generic type parameters. After all, this would be tremendously useful feature, I personally miss it greatly.</p> <p><strong>EDIT:</strong></p> <p>I would like to know of a hard-core issue, fixing which yields such a high price on implementing this feature that justifies it not being implemented yet. For instance, examine this fictional declaration:</p> <pre><code>class C&lt;T&gt; : T { } </code></pre> <p>As Eric Lippert has noticed what if "<strong>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 C?! What if T is a class dervied from C? 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?</strong>"</p> <p>As Eric continues, "<strong>Those are the easy, obvious ones</strong>". Indeed, he is right. I am interested in a concrete example of some neither easy nor obvious issue, which is hard to resolve.</p>
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    1. USJohn Saunders
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    1. COThere are connected reasons why C++ is more complicated than C#, and why you can do things in C++ that you can't do in C#. This is why, for the foreseeable future, some people will want to do some projects in something other than C#.
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      1. USDavid Thornley
    2. CORead those posts. I guess ROI is the reason: http://blogs.msdn.com/ericlippert/archive/2009/10/05/why-no-extension-properties.aspx and http://blogs.msdn.com/ericgu/archive/2004/01/12/57985.aspx
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      1. USAlex Reitbort
 

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