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    <p>My understanding is that it is not subtype relationships which are co/contra-variant but rather operations (or projections) between those types (such as delegates and generics). Therefore:</p> <pre><code>Animal someAnimal = new Giraffe(); </code></pre> <p>is not co-variant, but rather this is just assignment compatibility since the type Giraffe is 'smaller than' the type Animal. Co/contra-variance becomes an issue when you have some projection between these types, such as:</p> <pre><code>IEnumerable&lt;Giraffe&gt; giraffes = new[] { new Giraffe(); }; IEnumerable&lt;Animal&gt; animals = giraffes; </code></pre> <p>This is not valid in C#3, however it should be possible since a sequence of giraffes is a sequence of animals. The projection <code>T -&gt; IEnumerable&lt;T&gt;</code> preserves the 'direction' of the type relationship since <code>Giraffe &lt; Animal</code> and <code>IEnumerable&lt;Giraffe&gt; &lt; IEnumerable&lt;Animal&gt;</code> (note that assignment requires that the type of the left-hand side is at least as wide as the right).</p> <p>Contra-variance reverses the type relationship:</p> <pre><code>Action&lt;Animal&gt; printAnimal = a =&gt; {System.Console.WriteLine(a.Name)}; Action&lt;Giraffe&gt; printGiraffe = printAnimal; </code></pre> <p>This is also not legal in C#3, but it should be since any action taking an animal can cope with being passed a Giraffe. However, since <code>Giraffe &lt; Animal</code> and <code>Action&lt;Animal&gt; &lt; Action&lt;Giraffe&gt;</code> the projection has reversed the type relationships. This is legal in C#4.</p> <p>So to answer the questions in your example:</p> <pre><code>//the following are neither covariant or contravariant - since there is no projection this is just assignment compatibility Mammal mammal1 = new Giraffe(); Mammal mammal2 = new Dolphin(); //compare is contravariant with respect to its arguments - //the delegate assignment is legal in C#4 but not in C#3 Func&lt;Mammal, Mammal, bool&gt; compare = (m1, m2) =&gt; //whatever Func&lt;Giraffe, Dolphin, bool&gt; c2 = compare; //always invalid - right hand side must be smaller or equal to left hand side Mammal mammal1 = new Animal(); //not valid for same reason - animal cannot be assigned to Mammal Compare(new Animal(), new Dolphin()); </code></pre>
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    1. POContravariance explained
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    1. COLee, I of course understand the difference between assignment compatibility and co/contra-covariance. Introducing a layer in between such as a delegate of course explains the issue very clearly. One would assume that Action<Mammal> could be assigned to Action<Animal>, but of course this is wrong and only Action<Mammal> or less can be assigned to Action<Mammal>. Yes this sort of layer in the middle clearly shows contravariance, but only because there is a layer..are we saying that contravariance only happens when a layer such as this is introduced.
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    2. COi meant to say co/contra-variance... :), anyway I edited my question. Also i do believe that assignment compatibility is covariant in nature, because covariance preserves the assignment compatibility. However in terms of Action<Animal> = Action<Mammal>, this assignment compatibility is not preserved, but actually reversed..which is contravariance. Even tho Action<Animal> = Action<Mammal> is still assignment compatible (but only because declaring it as contravariant it would not be).
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