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    <p>You cannot do the... </p> <pre><code>ItemCollection&lt;Feature&gt; features = ...; ItemCollection&lt;Item&gt; items = features; </code></pre> <p>It's about generics <code>variance</code> (covariance and contravariance that is) - <strong>and it's only supported for interfaces, delegates - and only providing they're designed that way</strong> (decorated with <code>in</code> / <code>out</code> - and conforming to the rules that go with it). e.g. <code>IEnumerable&lt;&gt;</code> is (if you lookup its definition you'll see the <code>out</code>). For more details I think it's best to read more... </p> <p><a href="https://stackoverflow.com/questions/245607/how-is-generic-covariance-contra-variance-implemented-in-c-sharp-4-0/246101#246101">How is Generic Covariance &amp; Contra-variance Implemented in C# 4.0?</a><br> <a href="https://stackoverflow.com/questions/2719954/understanding-covariant-and-contravariant-interfaces-in-c-sharp/2720046#2720046">Understanding Covariant and Contravariant interfaces in C#</a><br> <a href="http://msdn.microsoft.com/en-us/library/dd799517.aspx" rel="nofollow noreferrer">http://msdn.microsoft.com/en-us/library/dd799517.aspx</a> </p> <p>In your case, you could design an <code>IItemCollection&lt;out T&gt;</code> (<code>interface</code>) - that (<strong>theoretically</strong>) might support the casting you need. But it'd have to be <code>read-only</code> (<em>a tad simplified and not entirely correct but it's easier to think that way - the rules are a bit more complex</em>). </p> <p>Since you're naming it a 'collection' I'm assuming it's not just for enumerating, viewing the items - i.e. if you have an <code>Add</code> of a sort (which requires <code>input parameters</code> - i.e. <code>contra-variance</code>) that clashes with the 'covariance' you need for the <code>upcasting</code>. Also any other parameters involved might not allow your interface to be made covariant. </p> <p><hr /> Also some other posts I made related... </p> <p><a href="https://stackoverflow.com/questions/15590540/how-to-make-generic-class-that-contains-a-set-of-only-its-own-type-or-subtypes-a/15599634#15599634">How to make generic class that contains a Set of only its own type or subtypes as Children?</a> </p> <p><a href="https://stackoverflow.com/questions/15723317/c-sharp-generic-handlers-what-am-i-misunderstanding/15723829#15723829">C# generic handlers, what am I misunderstanding?</a> </p>
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    1. POWhy is my derived class not converible to base class?
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    1. USCommunity
    1. USNSGaga
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    1. CO+1 for covariance and links. And I had no idea you could decorate type arguments with in / out.
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      1. USMalcolm O'Hare
    2. CO@MalcolmO'Hare you're welcome - yes, it could be useful in certain scenarios
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