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copied!<p>Variance will only be supported in a <em>safe</em> way - in fact, using the abilities that the CLR already has. So the examples I give in the book of trying to use a <code>List<Banana></code> as a <code>List<Fruit></code> (or whatever it was) still won't work - but a few other scenarios will.</p> <p>Firstly, it will only be supported for interfaces and delegates.</p> <p>Secondly, it requires the author of the interface/delegate to decorate the type parameters as <code>in</code> (for contravariance) or <code>out</code> (for covariance). The most obvious example is <code>IEnumerable<T></code> which only ever lets you take values "out" of it - it doesn't let you add new ones. That will become <code>IEnumerable<out T></code>. That doesn't hurt type safety at all, but lets you return an <code>IEnumerable<string></code> from a method declared to return <code>IEnumerable<object></code> for instance.</p> <p>Contravariance is harder to give concrete examples for using interfaces, but it's easy with a delegate. Consider <code>Action<T></code> - that just represents a method which takes a <code>T</code> parameter. It would be nice to be able to convert seamlessly use an <code>Action<object></code> as an <code>Action<string></code> - any method which takes an <code>object</code> parameter is going to be fine when it's presented with a <code>string</code> instead. Of course, C# 2 already has covariance and contravariance of delegates to some extent, but via an actual conversion from one delegate type to another (creating a new instance) - see P141-144 for examples. C# 4 will make this more generic, and (I believe) will avoid creating a new instance for the conversion. (It'll be a reference conversion instead.)</p> <p>Hope this clears it up a bit - please let me know if it doesn't make sense!</p>

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