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copied!<p>It's probably easiest to give examples - that's certainly how I remember them.</p> <p><strong>Covariance</strong></p> <p>Canonical examples: <code>IEnumerable<out T></code>, <code>Func<out T></code></p> <p>You can convert from <code>IEnumerable<string></code> to <code>IEnumerable<object></code>, or <code>Func<string></code> to <code>Func<object></code>. Values only come <em>out from</em> these objects.</p> <p>It works because if you're only taking values out of the API, and it's going to return something specific (like <code>string</code>), you can treat that returned value as a more general type (like <code>object</code>).</p> <p><strong>Contravariance</strong></p> <p>Canonical examples: <code>IComparer<in T></code>, <code>Action<in T></code></p> <p>You can convert from <code>IComparer<object></code> to <code>IComparer<string></code>, or <code>Action<object></code> to <code>Action<string></code>; values only go <em>into</em> these objects.</p> <p>This time it works because if the API is expecting something general (like <code>object</code>) you can give it something more specific (like <code>string</code>).</p> <p><strong>More generally</strong></p> <p>If you have an interface <code>IFoo<T></code> it can be covariant in <code>T</code> (i.e. declare it as <code>IFoo<out T></code> if <code>T</code> is only used in an output position (e.g. a return type) within the interface. It can be contravariant in <code>T</code> (i.e. <code>IFoo<in T></code>) if <code>T</code> is only used in an input position (e.g. a parameter type).</p> <p>It gets potentially confusing because "output position" isn't quite as simple as it sounds - a parameter of type <code>Action<T></code> is still only using <code>T</code> in an output position - the contravariance of <code>Action<T></code> turns it round, if you see what I mean. It's an "output" in that the values can pass from the implementation of the method <em>towards</em> the caller's code, just like a return value can. Usually this sort of thing doesn't come up, fortunately :)</p>

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