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POArray co-variance in C# generic list
 

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  1. POArray co-variance in C# generic list
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    <p>I have an example where I want an abstract class interface to return something like this</p> <pre><code>abstract class AnimalProcessor { public abstract IList&lt;Animal&gt; ProcessResults(); } </code></pre> <p>Then the concrete examples</p> <pre><code>class GiraffeProcessor : AnimalProcessor { public override IList&lt;Animal&gt; ProcessResults() { return new List&lt;Giraffe&gt;(); } } class LionProcessor : AnimalProcessor { public override IList&lt;Animal&gt; ProcessResults() { return new List&lt;Lion&gt;(); } } </code></pre> <p>The problem is that the concrete classes need to have the same signature to override the <code>ProcessResults()</code> method so they need to return an <code>IList&lt;Animal&gt;</code>, however the ACTUAL data I want to return is an <code>IList&lt;Lion&gt;</code>, <code>IList&lt;Giraffe&gt;</code> etc, but then the calling code has to do</p> <pre><code>GiraffeProcessor processor = new GiraffeProcessor(); IList&lt;Animal&gt; results = processor.GetResults(); </code></pre> <p>Which does not give me an Ilist which is what I want.</p> <p>Problems</p> <p>1) Above code does not compile. The giraffeProcessor has to return a concrete <code>List&lt;Animal&gt;</code>, you can populate it with <code>Giraffe</code> objects but the object type you construct to return has to be <code>List&lt;Animal&gt;</code>. Not ideal.</p> <p>2) When you return the results, you can only get an <code>IList&lt;Animal&gt;</code>, not <code>IList&lt;Giraffe&gt;</code>. I have tried casting explicitly to <code>IList&lt;Giraffe&gt;</code> with <code>IList&lt;Giraffe&gt; results = (IList&lt;Giraffe&gt;) processor.GetResults();</code> which gives a runtime error, presumably because the object returned is NOT an <code>IList&lt;Giraffe&gt;</code>, it is an <code>IList&lt;Animal&gt;</code> which CONTAINS <code>Giraffe</code> objects.</p> <p>Can anyone suggest what I am doing wrong here with my design as Im a bit stumped as to the best way to accomplish this.</p>
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    1. USMarc Gravell
    1. USNZJames
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    1. COWhat purpose does having a blank IList serve that having a typed IList doesn't serve?
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      1. USJoel Etherton
 

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