Refactoring Day 20 : Extract Subclass
Todays refactoring comes from Martin Fowlers catalog of patterns. You can find this refactoring in his catalog here
This refactoring is useful when you have methods on a base class that are not shared amongst all classes and needs to be pushed down into it’s own class. The example I’m using here is pretty straightforward. We start out with a single class called Registration. This class handles all information related to a student registering for a course.
1: public class Registration
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<pre><span class="lnum"> 2:</span> {</pre>
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<pre><span class="lnum"> 3:</span> <span class="kwrd">public</span> NonRegistrationAction Action { get; set; }</pre>
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<pre><span class="lnum"> 4:</span> <span class="kwrd">public</span> <span class="kwrd">decimal</span> RegistrationTotal { get; set; }</pre>
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<pre><span class="lnum"> 5:</span> <span class="kwrd">public</span> <span class="kwrd">string</span> Notes { get; set; }</pre>
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<pre><span class="lnum"> 6:</span> <span class="kwrd">public</span> <span class="kwrd">string</span> Description { get; set; }</pre>
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<pre><span class="lnum"> 7:</span> <span class="kwrd">public</span> DateTime RegistrationDate { get; set; }</pre>
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<pre><span class="lnum"> 8:</span> }</pre>
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There is something that we’ve realized after working with this class. We are using it in two different contexts. The properties NonRegistrationAction and Notes are only ever used when dealing with a NonRegistration which is used to track a portion of the system that is slightly different than a normal registration. Noticing this, we can extract a subclass and move those properties down into the NonRegistration class where they more appropriately fit.
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<pre><span class="lnum"> 1:</span> <span class="kwrd">public</span> <span class="kwrd">class</span> Registration</pre>
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<pre><span class="lnum"> 2:</span> {</pre>
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<pre><span class="lnum"> 3:</span> <span class="kwrd">public</span> <span class="kwrd">decimal</span> RegistrationTotal { get; set; }</pre>
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<pre><span class="lnum"> 4:</span> <span class="kwrd">public</span> <span class="kwrd">string</span> Description { get; set; }</pre>
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<pre><span class="lnum"> 5:</span> <span class="kwrd">public</span> DateTime RegistrationDate { get; set; }</pre>
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<pre><span class="lnum"> 6:</span> }</pre>
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<pre><span class="lnum"> 7:</span> </pre>
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<pre><span class="lnum"> 8:</span> <span class="kwrd">public</span> <span class="kwrd">class</span> NonRegistration : Registration</pre>
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<pre><span class="lnum"> 9:</span> {</pre>
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<pre><span class="lnum"> 10:</span> <span class="kwrd">public</span> NonRegistrationAction Action { get; set; }</pre>
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<pre><span class="lnum"> 11:</span> <span class="kwrd">public</span> <span class="kwrd">string</span> Notes { get; set; }</pre>
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<pre><span class="lnum"> 12:</span> }</pre>
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<em><span style="font-size: xx-small">This is part of the 31 Days of Refactoring series. For a full list of Refactorings please see the <a href="/blogs/sean_chambers/archive/2009/08/01/31-days-of-refactoring.aspx" target="_blank">original introductory post</a>.</span></em>
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