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    <blockquote> <p>Can someone explain the usage of the out T part with an example?</p> </blockquote> <p>Sure. <code>IEnumerable&lt;T&gt;</code> is covariant. That means you can do this:</p> <pre><code>static void FeedAll(IEnumerable&lt;Animal&gt; animals) { foreach(Animal animal in animals) animal.Feed(); } ... IEnumerable&lt;Giraffe&gt; giraffes = GetABunchOfGiraffes(); FeedAll(giraffes); </code></pre> <p>"Covariant" means that <em>the assignment compatibility relationship of the type argument is preserved in the generic type</em>. <code>Giraffe</code> is assignment compatible with <code>Animal</code>, and therefore that relationship is preserved in the constructed types: <code>IEnumerable&lt;Giraffe&gt;</code> is assignment compatible with <code>IEnumerable&lt;Animal&gt;</code>.</p> <blockquote> <p>Why is applicable only for interfaces and delegates and not for classes?</p> </blockquote> <p>The problem with classes is that classes tend to have mutable fields. Let's take an example. Suppose we allowed this:</p> <pre><code>class C&lt;out T&gt; { private T t; </code></pre> <p>OK, now think this question through carefully before you go on. <strong>Can <code>C&lt;T&gt;</code> have any method outside of the constructor that sets the field <code>t</code> to something other than its default?</strong></p> <p>Because it must be typesafe, <code>C&lt;T&gt;</code> can now have no methods that take a T as an argument; T can only be returned. So who sets t, and <em>where do they get the value they set it from</em>?</p> <p>Covariant class types really only work if the class is <em>immutable</em>. And we don't have a good way to make immutable classes in C#.</p> <p>I wish we did, but we have to live with the CLR type system that we were given. I hope in the future we can have better support for both immutable classes, and for covariant classes.</p> <p>If this feature interests you, consider reading my long series on how we designed and implemented the feature. Start from the bottom:</p> <p><a href="https://blogs.msdn.microsoft.com/ericlippert/tag/covariance-and-contravariance/" rel="noreferrer">https://blogs.msdn.microsoft.com/ericlippert/tag/covariance-and-contravariance/</a></p>
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    1. COThanks Eric! That gives me the general idea and I'll read through your blog posts. PS: When I saw an answer from you, I was hoping for to see some hotel rooms and stolen keys but I will take Giraffes.
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    2. COIf `C<T>` has multiple fields whose types involve `T`, it could have methods which set `t` based upon information in another field. Further, even if fields could only be set in a constructor, it would be useful if something like a `Tuple<Cat, ToyotaPrius>` could be passed to code expecting a `Tuple<Animal, Car>`. I think a more a fundamental problem with covariant classes is that every different class produced form `Foo<T>` has its own set of static variables. If `Foo<T>.Q()` was an instance property that returned a static field, and one could have `Foo<Animal> it = new Foo<Cat>();`...
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    3. CO...which class' static field should be used to evaluate `it.Q()`? It would seem strange for method `Q` of a `Foo<Animal>` to actually bind to an instance method of class `Foo<Cat>`.
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