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PODependency Injection and Runtime Object Creation
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copied!<p>I've been trying to follow the principles of Dependency Injection, but <A HREF="http://misko.hevery.com/2008/10/21/dependency-injection-myth-reference-passing/" rel="nofollow noreferrer">after reading this article, I know I'm doing something wrong.</A></p> <p>Here's my situation: My application receives different types of physical mail. All the incoming mail passes through my <code>MailFunnel</code> object.</p> <p>While it's running, <code>MailFunnel</code> receives different types of messages from the outside: Box, Postcard and Magazine. </p> <p>Each mail type needs to be handled differently. For example, if a Box comes in, I may need to record the weight before delivering it. Consequently, I have <code>BoxHandler</code>, <code>PostcardHandler</code> and <code>MagazineHandler</code> objects.</p> <p>Each time a new message comes into my <code>MailFunnel</code>, I instantiate a <b>new</b> corresponding <code>MailHandler</code> object. </p> <p>For example:</p> <pre> class MailFunnel { void NewMailArrived( Mail mail ) { switch (mail.type) { case BOX: BoxHandler * bob = new BoxHandler(shreddingPolicy, maxWeightPolicy); bob->get_to_work(); break; case POSTCARD: PostcardHandler * frank = new PostcardHandler(coolPicturePolicy); frank->get_to_work(); break; case MAGAZINE: MagazineHandler * nancy = new MagazineHandler(censorPolicy); nancy->get_to_work(); break; } } private: MaxWeightPolcy & maxWeightPolicy; ShreddingPolicy & shreddingPolicy; CoolPicturePolicy & coolPicturePolicy; CensorPolicy & censorPolicy; } </pre> <p>On one hand, this is great because it means that if I get five different pieces of mail in, I immediately have five different <code>MailHandlers</code> working concurrently to take care of business. However, this also means that <a href="http://misko.hevery.com/2008/09/10/where-have-all-the-new-operators-gone/" rel="nofollow noreferrer">I'm mixing object creation with application logic</a> - a big no-no when it comes to Dependency Injection.</p> <p>Also, I have all these policy references hanging around in my <code>MailFunnel</code> object that <code>MailFunnel</code> really doesn't need. The only reason that <code>MailFunnel</code> has these objects is to pass them to the <code>MailHandler</code> constructors. Again, <A HREF="http://misko.hevery.com/2008/10/21/dependency-injection-myth-reference-passing/" rel="nofollow noreferrer">this is another thing I want to avoid</A>.</p> <p>All recommendations welcome. Thanks!</p>

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