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POShould I support Co-/Contravariance for Pub-/Sub-Scenarios in C#3.0?
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  1. POShould I support Co-/Contravariance for Pub-/Sub-Scenarios in C#3.0?
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    copied!<p>In my application I am creating a simple event hub that offers something for registering subscribers:</p> <pre><code>Subscribes&lt;EventType&gt;(ISubscriber&lt;EventType&gt; subscriber) // and some other methods for adding subscribers </code></pre> <p>And for publishing events.</p> <pre><code>Publish&lt;EventType&gt;(EventType @event) </code></pre> <p>Quite simple. I want to route <code>Publish&lt;int&gt;(0)</code> to all subscribers implementing <code>ISubscriber&lt;int&gt;</code>.</p> <p>What's not too hard is, that I want the subscribers EventType to be contravariant. So <code>ISubscriber&lt;object&gt;</code> should basically consume everything. Not shure wether I want them to consume valuetypes, too.</p> <p>With C#4 that is no problem, but now I'm doing this stuff with C#3 and just faking contravariance with an interface:</p> <pre><code>public interface IContravariantGenerics { object AsVariantFor(Type[] genericParamters); } </code></pre> <p>Well, now, I want to empack data into "event types" like this. The generic parameters of that events must be covariant.</p> <pre><code>SubX : ISubscriber&lt;DataChanged&lt;A&gt;&gt; DataChanged&lt;T&gt; T Data {get;} </code></pre> <p>When I publish <code>Publish&lt;DataChanged&lt;B&gt;&gt;(new DataChanged&lt;B&gt;(new B())</code> (given B : A), the Subscriber should be notified with <code>DataChanged&lt;A&gt;</code> where Data is the B-instance passed to <code>DataChanged&lt;B&gt;</code>. So I need covariance support as well.</p> <p>I thought of writing a lib that supports both Co-/And Contravariance like this:</p> <pre><code>IMyObject&lt;T1, T2&gt; : IWithVariance&lt;In, Out&gt; </code></pre> <p>Which would allow conversions (not casts!) like this:</p> <pre><code>Obj&lt;Fruit, Fruit&gt; x; IMyObject&lt;Apple, object&gt; x2 = x.ToVariant&lt;Apple, object&gt;(); </code></pre> <p>What do you think? Is it possible? Thought of doing it using dynamic proxies.</p>
 

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