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    <p>In this part:</p> <blockquote> <p>When I publish <code>Publish&lt;DataChanged&lt;B&gt;&gt;(new DataChanged&lt;B&gt;(new B())</code>, the <code>Subscriber</code> should be notified with <code>DataChanged&lt;A&gt;</code> where <code>.Data</code> is the <code>B</code>-instance.</p> </blockquote> <p>I may not have understood you - I can't see what <code>.Data</code> refers to, for example, and I can only guess at the relationship between B and A. Do you mean that B is derived from A?</p> <p>If so, C# 4 will not necessarily make such a thing happen automatically. The types <code>X&lt;A&gt;</code> and <code>X&lt;B&gt;</code> are not compatible at all by default. If <code>X</code> is an <code>interface</code> and the type parameter is marked as <code>out</code>, then <code>X&lt;B&gt;</code> can be assigned to a variable of type <code>X&lt;A&gt;</code>. But note that this is for interfaces only, not concrete types (there is a similar provision for delegates, but that's all).</p> <p><strong>Edit:</strong> </p> <p>So therefore what you want to do is simulate the way that <code>X&lt;B&gt;</code> can be assigned to a variable of type <code>X&lt;A&gt;</code> in C#/CLR 4.0, where X is an interface.</p> <p>Suppose X is:</p> <pre><code>interface X&lt;T&gt; { T Foo(int arg); // Note: T may only appear as an output, so this is illegal: // void Foo(T arg); } </code></pre> <p>You have an <code>X&lt;B&gt;</code>, you need an <code>X&lt;A&gt;</code>. You know that <code>B</code> is assignable to <code>A</code>. So you need the following adaptor:</p> <pre><code>class WrapX_A_B : X&lt;A&gt; { public X&lt;B&gt; Impl { get; set; } public A Foo(int arg) { return Impl.Foo(arg); } } </code></pre> <p>You just forward each method on to the real implementation.</p> <p>However, you would need such a wrapper class for every possible combination of generic outer classes and pairs of generic parameters related by inheritance. It would be a tedious, error-prone and never-complete task to write them all by hand and maintain a big lookup to pick the right one for a given situation.</p> <p>So now you're into code generation to manufacture the wrapper classes at runtime.</p>
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    1. POShould I support Co-/Contravariance for Pub-/Sub-Scenarios in C#3.0?
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    1. USDaniel Earwicker
    1. USDaniel Earwicker
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    1. COYou're right. Corrected the question. In C#4 I would end up with ISubscriber<in T> and IChangedEvent<out T>.
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      1. USLars Corneliussen
    2. COSee update above.
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      1. USDaniel Earwicker
 

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