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copied!<p>Basically, variance applies when the CLR can ensure that it doesn't need to make any <em>representational change</em> to the values. References all look the same - so you can use an <code>IEnumerable<string></code> as an <code>IEnumerable<object></code> without any change in representation; the native code itself doesn't need to know what you're doing with the values at all, so long as the infrastructure has guaranteed that it will definitely be valid.</p> <p>For value types, that doesn't work - to treat an <code>IEnumerable<int></code> as an <code>IEnumerable<object></code>, the code using the sequence would have to know whether to perform a boxing conversion or not.</p> <p>You might want to read Eric Lippert's <a href="http://blogs.msdn.com/b/ericlippert/archive/2009/03/19/representation-and-identity.aspx">blog post on representation and identity</a> for more on this topic in general.</p> <p>EDIT: Having reread Eric's blog post myself, it's at least as much about <em>identity</em> as representation, although the two are linked. In particular:</p> <blockquote> <p>This is why covariant and contravariant conversions of interface and delegate types require that all varying type arguments be of reference types. To ensure that a variant reference conversion is always identity-preserving, all of the conversions involving type arguments must also be identity-preserving. The easiest way to ensure that all the non-trivial conversions on type arguments are identity-preserving is to restrict them to be reference conversions.</p> </blockquote>

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