title: Design pattern for cost calculator app?
body: <p>I have a problem that I’ve tried to get help for before, but I wasn’t able to solve it then, so I’m trying to simplify the problem now to see if I can get some more concrete help with this because it is driving me crazy…</p>
  
  <p>Basically, I have a working (more complex) version of this application, which is a project cost calculator. But because I am at the same time trying to learn to design my applications better, I would like some input on how I could improve this design. Basically the main thing I want is input on the conditionals that (here) appear repeated in two places. The suggestions I got before was to use the strategy pattern or factory pattern. I also know about the Martin Fowler book with the suggestion to Refactor conditional with polymorphism. I understand that principle in his simpler example. But how can I do either of these things here (if any would be suitable)? The way I see it, the calculation is dependent on a couple of conditions: 1. What kind of service is it, writing or analysis? 2. Is the project small, medium or large? (Please note that there may be other parameters as well, equally different, such as “are the products new or previously existing?” So such parameters should be possible to add, but I tried to keep the example simple with only two parameters to be able to get concrete help)</p>
  
  <p>So refactoring with polymorphism would imply creating a number of subclasses, which I already have for the first condition (type of service), and should I really create more subclasses for the second condition as well (size)? What would that become, AnalysisSmall, AnalysisMedium, AnalysisLarge, WritingSmall, etc…??? No, I know that’s not good, I just don’t see how to work with that pattern anyway else?</p>
  
  <p>I see the same problem basically for the suggestions of using the strategy pattern (and the factory pattern as I see it would just be a helper to achieve the polymorphism above). So please, if anyone has concrete suggestions as to how to design these classes the best way I would be really grateful! Please also consider whether I have chosen the objects correctly too, or if they need to be redesigned. (Responses like "you should consider the factory pattern" will obviously not be helpful... I've already been down that road and I'm stumped at precisely how in this case)</p>
  
  <p>Regards,</p>
  
  <p>Anders</p>
  
  <p>The code (very simplified, don’t mind the fact that I’m using strings instead of enums, not using a config file for data etc, that will be done as necessary in the real application once I get the hang of these design problems):</p>
  
  <pre><code>public abstract class Service
  {
      protected Dictionary&lt;string, int&gt; _hours;
      protected const int SMALL = 2;
      protected const int MEDIUM = 8;
  
      public int NumberOfProducts { get; set; }
      public abstract int GetHours();
  }
  
  public class Writing : Service
  {
      public Writing(int numberOfProducts)
      {
          NumberOfProducts = numberOfProducts;
          _hours = new Dictionary&lt;string, int&gt; { { "small", 125 }, { "medium", 100 }, { "large", 60 } };
      }
  
      public override int GetHours()
      {
          if (NumberOfProducts &lt;= SMALL)
              return _hours["small"] * NumberOfProducts;
          if (NumberOfProducts &lt;= MEDIUM)
              return (_hours["small"] * SMALL) + (_hours["medium"] * (NumberOfProducts - SMALL));
          return (_hours["small"] * SMALL) + (_hours["medium"] * (MEDIUM - SMALL))
              + (_hours["large"] * (NumberOfProducts - MEDIUM));
      }
  }
  
  public class Analysis : Service
  {
      public Analysis(int numberOfProducts)
      {
          NumberOfProducts = numberOfProducts;
          _hours = new Dictionary&lt;string, int&gt; { { "small", 56 }, { "medium", 104 }, { "large", 200 } };
      }
  
      public override int GetHours()
      {
          if (NumberOfProducts &lt;= SMALL)
              return _hours["small"];
          if (NumberOfProducts &lt;= MEDIUM)
              return _hours["medium"];
          return _hours["large"];
      }
  }
  
  public partial class Form1 : Form
  {
      public Form1()
      {
          InitializeComponent();
          List&lt;int&gt; quantities = new List&lt;int&gt;();
  
          for (int i = 0; i &lt; 100; i++)
          {
              quantities.Add(i);
          }
          comboBoxNumberOfProducts.DataSource = quantities;
      }
  
      private void comboBoxNumberOfProducts_SelectedIndexChanged(object sender, EventArgs e)
      {
          Service writing = new Writing((int) comboBoxNumberOfProducts.SelectedItem);
          Service analysis = new Analysis((int) comboBoxNumberOfProducts.SelectedItem);
  
          labelWriterHours.Text = writing.GetHours().ToString();
          labelAnalysisHours.Text = analysis.GetHours().ToString();
      }
  }
  </code></pre>