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copied!<p>It's not a stupid question, it's a good one. :)</p> <p>What you're asking for on the interface is commonly referred to as "<a href="http://en.wikipedia.org/wiki/Duck_typing" rel="nofollow noreferrer">duck-typing</a>." It isn't supported right now, but C#4.0 will support it via the new "<a href="https://stackoverflow.com/questions/301479/show-me-the-way-to-use-new-dynamic-keyword-in-c-4-0"><code>dynamic</code></a>" keyword.</p> <p>You've really got three choices that I'm aware of at this point:</p> <ol> <li><p>You can go up the tree until you find the common ancestor (probably <a href="http://msdn.microsoft.com/en-us/library/system.componentmodel.component.aspx" rel="nofollow noreferrer"><code>Component</code></a>) and then downcast to your supported types. If the downcast fails, you throw or handle appropriately. </p> <p><strong>Pro:</strong> Minimal code duplication.</p> <p><strong>Con:</strong> You're trading away compile-time type safety for runtime type safety. You have to add error checking/handling for invalid casts.</p> <p><strong>Code:</strong></p> <pre><code>public void UseThumbnail(Component c) { PictureBox p = c as PictureBox; if(p != null) // do whatever // so forth } </code></pre></li> <li><p>You can duplicate functionality as appropriate for everything that you need to implement this functionality for. </p> <p><strong>Pro:</strong> Maintain compile time type safety</p> <p><strong>Con:</strong> You're duplicating code for different types. This can grow into a significant maintenance burden, especially if you're handling more than two similar classes.</p> <p><strong>Code:</strong></p> <pre><code>public void UsePictureBox(PictureBox p) { // Some code X } public void UseOtherControl(OtherControl p) { // Some code X } </code></pre></li> <li><p>You can create your special interface and subclass the classes you want to support to expose that common functionality.</p> <p><strong>Pro:</strong> You get compile time safety and can program against your new interface.</p> <p><strong>Con:</strong> You have to add an empty subclass for everything you're dealing with, and you need to use them.</p> <p><strong>Code:</strong></p> <pre><code>public class ThumbnailPictureBox : PictureBox, IThumbnail { } </code></pre></li> </ol>

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