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POExtension methods on base generic interfaces
 

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    <p>I'm implementing a fluent builder pattern that requires accepting enumerables in a static extension method and iterating through its contents, while applying a functor to the enumerable's contents. As in (not the actual code, just an illustration):</p> <pre><code>public static IValidator&lt;IEnumerable&lt;T&gt;&gt; Each&lt;T&gt;( this IValidator&lt;IEnumerable&lt;T&gt;&gt; enumerable, Func&lt;T, bool&gt; action) { foreach (T value in enumerable) action(value); return validator; } </code></pre> <p>This works pretty well for enumerables but fails for inherited types/interfaces. Let's say:</p> <pre><code>IValidator&lt;IEnumerable&lt;Guid&gt;&gt; validator = ...; IEnumerable&lt;Guid&gt; guids = ...; validator.Each(guids, guid =&gt; guid != Guid.Empty); // ok IList&lt;Guid&gt; guids = ...; validator.Each(guids, guid =&gt; guid != Guid.Empty); // doesn't compile (see below) </code></pre> <p>The exception is:</p> <blockquote> <p><code>IValidator&lt;IList&lt;Guid&gt;&gt;</code> does not contain a definition for 'Each' and no extension method 'Each' accepting a first argument of type <code>IValidator&lt;IList&lt;Guid&gt;&gt;</code> could be found (are you missing a using directive or an assembly reference?</p> </blockquote> <p>My question is about the inheritance chain of <code>IValidator&lt;T&gt;</code> and, more specifically, its generic type arguments <code>T</code>. Why is type <code>IValidator&lt;IEnumerable&lt;T&gt;&gt;</code> not assignable from <code>IValidator&lt;IList&lt;T&gt;&gt;</code>? There's no circumstance I can think of on which <code>IList&lt;T&gt;</code> is not an <code>IEnumerable&lt;T&gt;</code> (given the same <code>T</code>).</p> <p>Constraining the generic argument to <code>T : IEnumerable&lt;R&gt;</code> does work but that requires two type arguments (<code>T</code> and <code>R</code>) which I'd like to avoid, if possible.</p> <p>Any thoughts? Better solutions? Thanks. </p>
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    1. COI think this could be a covariance and contravariance issue. Let me see if I can get some sample code to see if this the case.
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      1. USJason Evans
    2. COEven if you solve your current problem, what's the point of the method? You aren't using the boolean result of the functor passed in, so unless the functor mutates the object the net result is nothing changes, and if it does mutate it then...well...it shouldn't, as Linq is designed for queries, not mutation, of sequences.
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    3. CO@Servy - that's just a sample to illustrate my intentions, not the actual code. I left it as `Func<T, bool>` because it requires less code than `Action<T>` to write the calling lambdas in the example that follows. I was being a little lazy. :-)
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