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POHow to apply a generic extension to a specific generic type?
 

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  1. POHow to apply a generic extension to a specific generic type?
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    <p>Imagine i have an extension method that operates of an interface of abstract objects:</p> <pre><code>public static void BlowTheirWhoHoovers(this ICollection&lt;Image&gt; source) { ... } </code></pre> <p>This extension knows how to blow the who hoovers of a list of images.</p> <p><strong>Note</strong>:</p> <ul> <li>it's a list of <strong>Images</strong>, which is abstract. As long as the lists contain anything that descends from Image (e.g. <code>Bitmap</code>, <code>Metafile</code>, <code>PngImage</code>) the extension can handle it</li> <li><code>source</code> list can by any kind of list as long as it exposes <code>ICollection</code></li> <li>since <code>source</code> list is going to potentially have items added or removed, it is <code>ICollection</code> rather than <code>IEnumeration</code> (<code>IEnumeration</code> doesn't support modifying the list)</li> </ul> <p>That's all well and good, except it doesn't work, the extension method is not found:</p> <pre><code>Collection&lt;Bitmap&gt; whoHonkers = new Collection&lt;Bitmap&gt;(); whoHonkers.BlowTheirWhoHoovers(); </code></pre> <p><img src="https://i.stack.imgur.com/LlMRo.png" alt="enter image description here"></p> <p>i assume this fails because: </p> <ul> <li>Visual Studio doesn't think <code>Collection</code> implements <code>ICollection&lt;T&gt;</code></li> <li>Visual Studio doesn't think <code>Bitmap</code> descends from <code>Image</code></li> </ul> <p>Now i <em>can</em> change my extension:</p> <pre><code>public static void BlowTheirWhoHoovers(this Collection&lt;Bitmap&gt; source) { ... } </code></pre> <p>which <em>does</em> compile:</p> <p><img src="https://i.stack.imgur.com/JLDIR.png" alt="enter image description here"></p> <p>except it's no longer useful (it requires <code>Collection</code>, and it requires <code>Bitmap</code>), e.g. it fails to work on anything except <code>Collection&lt;T&gt;</code>:</p> <pre><code>List&lt;Bitmap&gt; whoHonkers = new List&lt;Bitmap&gt;(); whoHonkers.BlowTheirWhoHoovers(); </code></pre> <p>It also fails on anything that it's <code>Bitmap</code>:</p> <pre><code>List&lt;Metafile&gt; whoHonkers = new List&lt;Metafile&gt;(); whoHonkers.BlowTheirWhoHoovers(); </code></pre> <p>or </p> <pre><code>List&lt;PngImage&gt; whoHonkers = new List&lt;PngImage&gt;(); whoHonkers.BlowTheirWhoHoovers(); </code></pre> <p>How can i apply an extension to anything that <strong>is</strong>:</p> <pre><code>ICollection&lt;Image&gt; </code></pre> <p>?</p> <p>i can't force anyone to change their declaration, e.g.:</p> <pre><code>List&lt;Bitmap&gt; </code></pre> <p>to</p> <pre><code>Collection&lt;Image&gt; </code></pre> <p><strong>Update</strong>:</p> <p>Possible avenues of solution exploration: </p> <ul> <li>finding the syntax that allows extensions on things that the compiler should be able to do</li> <li>manually casting the list; doing the compiler's job for it</li> </ul> <p><strong>Update#2</strong>:</p> <p>i could have simplified some of the distractions around <code>ICollection</code>, and made it <code>IEnumerable</code> (since it's a hypothetical example i can make up whatever i want). But the screenshots already show <code>Collection</code>, so i'm going to leave it as it is.</p> <p><strong>Update#3</strong>: </p> <p>As <a href="https://stackoverflow.com/a/8658578/12597">foson pointed out</a>, changing it to <code>IEnumerable</code> allows a way to cheat the answer. So it definitely should stay as <code>ICollection</code>.</p>
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