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    <blockquote> <p>Now, to my untrained eye, covariance seems to be the same as upcasting, except that it refers the casting of collections. (And of a similar statement can be made regarding contravariance and downcasting).</p> <p>Is it really that simple?</p> </blockquote> <p>Covariance isn't about upcasting, although I can see why you think it's related.</p> <p>Covariance is about the following very simple idea. Let's say you have a variable <code>derivedSequence</code> of type <code>IEnumerable&lt;Derived&gt;</code>. Let's say you have a variable <code>baseSequence</code> of type <code>IEnumerable&lt;Base&gt;</code>. Here, <code>Derived</code> derives from <code>Base</code>. Then, with covariance, the following is a legal assignment, and an implicit reference conversion occurs:</p> <pre><code>baseSequence = derivedSequence; </code></pre> <p>Note that this is not upcasting. It is not the case that <code>IEnumerable&lt;Derived&gt;</code> derives from <code>IEnumerable&lt;Base&gt;</code>. Rather, it is covariance that allows you to assign the value of the variable <code>derivedSequence</code> to the variable <code>baseSequence</code>. The idea is that variables of type <code>Base</code> can be assigned from objects of type <code>Derived</code>, and since <code>IEnumerable&lt;T&gt;</code> is covariant in its parameter, objects of type <code>IEnumerable&lt;Derived&gt;</code> can be assigned to variables of type <code>IEnumerable&lt;Base&gt;</code>.</p> <p>Of course, I haven't yet really explained what covariance is. In general, covariance is about the following simple idea. Let's say you have a mapping <code>F</code> from types to types (I'll denote this mapping by <code>F&lt;T&gt;</code>; given a type <code>T</code> its image under the mapping <code>F</code> is <code>F&lt;T&gt;</code>.) Let's say that this mapping has the following very special property:</p> <blockquote> <p>if <code>X</code> is assignment compatible with <code>Y</code>, then <code>F&lt;X&gt;</code> is assignment compatible with <code>F&lt;Y&gt;</code> as well.</p> </blockquote> <p>In this case, we say that <code>F</code> is covariant in its parameter <code>T</code>. (Here, to say that "<code>A</code> is assignment compatible with <code>B</code>" where <code>A</code> and <code>B</code> are reference types means that instances of <code>B</code> can be stored in variables of type <code>A</code>.)</p> <p>In our case, <code>IEnumerable&lt;T&gt;</code> in C# 4.0, an implicit reference conversion from instances of <code>IEnumerable&lt;Derived&gt;</code> to <code>IEnumerable&lt;Base&gt;</code> if <code>Derived</code> is derived from <code>Base</code>. The direction of assignment compatibility is preserved, and this is why we say that <code>IEnumerable&lt;T&gt;</code> is covariant in its type parameter.</p>
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    1. PODifference between covariance and upcasting
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    1. USAlex Bagnolini
    1. USjason
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    1. COThus far this is the only answer that actually explains the difference, so nicely done.
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      1. USEric Lippert
    2. COYeah, wonderful answer, deserving at least as many votes as the question currently has. Hence the accept that I shall now perform.
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      1. USJames Wiseman
    3. CO...That by no means denigrates the other answers, of course :-)
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