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POUsing base class as generic for IEnumerable<T>
 

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    <p>I have a good understanding of OOP in general, inheritance and polymorphism, interfaces, etc. I encountered a strange situation and I don't understand why it does not work at all...</p> <p>EDIT : Ok, I found out that covariance (or contravariance?) may solve this problem, but crucially</p> <h2>we're still using .NET 2.0 </h2> <p>How can I solve this without moving to C# 4.0 ?</p> <p>Here is the situation. Given these two classes :</p> <pre><code>public class CustomCollectionType&lt;T&gt; : IEnumerable&lt;T&gt; { /* Implementation here, not really important */ } public class Entity : EntityBase { /* Implentation here, not important */ } </code></pre> <p>The compiler complains when I try to have this generic method </p> <pre><code>public void LoopThrough(IEnumerable&lt;EntityBase&gt; entityList) { foreach(EntityBase entity in entityList) { DoSomething(entity); } } </code></pre> <p>And try to use it this way :</p> <pre><code>CustomCollectionType&lt;Entity&gt; entityList; /* Add items to list */ LoopThrough(entityList); </code></pre> <p>Error says I cannot convert from <code>CustomCollectionType&lt;Entity&gt;</code> to <code>IEnumerable&lt;EntityBase&gt;</code>.</p> <p>However, I can do this :</p> <pre><code>public void Foo(EntityBase entity) { entity.DoSomething(); } Foo(new Entity()); </code></pre> <p>And this :</p> <pre><code>public void Bar(IEnumerable&lt;Entity&gt; entityList) { ... } CustomCollectionType&lt;Entity&gt; entityList; Bar(entityList); </code></pre> <p>Why can't I create my method with the highest classes in the hierarchy? The types are obviously compatible... Am I missing something ?</p> <p>EDIT : I want to solve this problem without altering the existing classes in any way, so creating a new method in any of the classes, or implementing an additional interface is out of the question.</p>
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    1. COLippert has a long series of post describing this topic. First part can be found at http://blogs.msdn.com/b/ericlippert/archive/2007/10/16/covariance-and-contravariance-in-c-part-one.aspx
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    2. COAnswer in dieter and dieter Java programming series
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