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    <p>Your <code>GetDiscount</code> method is classic example of <a href="http://en.wikipedia.org/wiki/Open/closed_principle" rel="nofollow">Open/Closed principle</a> violation. When you add new book type you have to add new <code>if</code> block to <code>GetDiscount</code>.</p> <p>Better way is to use some existing techinques which allow you to add new functionality without necessity to modify existing code. For example, <a href="http://en.wikipedia.org/wiki/Composite_pattern" rel="nofollow">Composite pattern</a>. I'll write some draft implementation of composite discount evaluator. You can add new discount evaluators based on any book proerties (date, price, etc.) easily.</p> <p>Also, I'll use interfaces instead of inheritance. Inheritance is a very strong link between two entities and in this case it is excessive.</p> <p>Listing is 167 lines long, so here is more comfort <a href="http://pastebin.com/SyM0cK6u" rel="nofollow">pastebin copy</a></p> <pre><code>using System; using System.Collections.Generic; using System.Linq; namespace ConsoleApplication1 { class Program { static void Main() { var compositeDiscountEvaluator = ConfigureEvaluator(); var scienceBook = new TextBook { Date = DateTime.Now, Price = 100, Genres = new[] {TextBooksGenre.Math} }; var textBook = new TextBook { Date = DateTime.Now, Price = 100, Genres = new[] {TextBooksGenre.Math, TextBooksGenre.Science} }; var fictionBook = new ReadingBook { Date = DateTime.Now, Price = 200, Genres = new[] {ReadingBooksGenre.Fiction} }; var readingBook = new ReadingBook { Date = DateTime.Now, Price = 300, Genres = new[] {ReadingBooksGenre.Fiction, ReadingBooksGenre.NonFiction} }; Console.WriteLine(compositeDiscountEvaluator.GetDiscount(scienceBook)); Console.WriteLine(compositeDiscountEvaluator.GetDiscount(textBook)); Console.WriteLine(compositeDiscountEvaluator.GetDiscount(fictionBook)); Console.WriteLine(compositeDiscountEvaluator.GetDiscount(readingBook)); } private static IDiscountEvaluator ConfigureEvaluator() { var evaluator = new CompositeDiscountEvaluator(); evaluator.AddEvaluator(new ReadingBookDiscountEvaluator()); evaluator.AddEvaluator(new TextBookDiscountEvaluator()); return evaluator; } } class CompositeDiscountEvaluator : IDiscountEvaluator { private readonly ICollection&lt;IDiscountEvaluator&gt; evaluators; public CompositeDiscountEvaluator() { evaluators = new List&lt;IDiscountEvaluator&gt;(); } public void AddEvaluator(IDiscountEvaluator evaluator) { evaluators.Add(evaluator); } public bool CanEvaluate&lt;TGenre&gt;(IBook&lt;TGenre&gt; book) { return evaluators.Any(e =&gt; e.CanEvaluate(book)); } public int GetDiscount&lt;TGenre&gt;(IBook&lt;TGenre&gt; book) { if (!CanEvaluate(book)) throw new ArgumentException("No suitable evaluator"); return evaluators.Where(e =&gt; e.CanEvaluate(book)).Select(e =&gt; e.GetDiscount(book)).Max(); } } interface IDiscountEvaluator { bool CanEvaluate&lt;TGenre&gt;(IBook&lt;TGenre&gt; book); int GetDiscount&lt;TGenre&gt;(IBook&lt;TGenre&gt; book); } class ReadingBookDiscountEvaluator : IDiscountEvaluator { private readonly IDictionary&lt;ReadingBooksGenre, int&gt; discounts; public ReadingBookDiscountEvaluator() { discounts = new Dictionary&lt;ReadingBooksGenre, int&gt; { {ReadingBooksGenre.Fiction, 3}, {ReadingBooksGenre.NonFiction, 4} }; } public bool CanEvaluate&lt;TGenre&gt;(IBook&lt;TGenre&gt; book) { return book is ReadingBook; } public int GetDiscount&lt;TGenre&gt;(IBook&lt;TGenre&gt; book) { var readingBook = (ReadingBook) book; return readingBook.Genres.Select(g =&gt; discounts[g]).Max(); } } class TextBookDiscountEvaluator : IDiscountEvaluator { private readonly IDictionary&lt;TextBooksGenre, int&gt; discounts; public TextBookDiscountEvaluator() { discounts = new Dictionary&lt;TextBooksGenre, int&gt; { {TextBooksGenre.Math, 1}, {TextBooksGenre.Science, 2} }; } public bool CanEvaluate&lt;TGenre&gt;(IBook&lt;TGenre&gt; book) { return book is TextBook; } public int GetDiscount&lt;TGenre&gt;(IBook&lt;TGenre&gt; book) { var textBook = (TextBook) book; return textBook.Genres.Select(g =&gt; discounts[g]).Max(); } } interface IBook&lt;TGenre&gt; { int Price { get; set; } DateTime Date { get; set; } TGenre[] Genres { get; set; } } class ReadingBook : IBook&lt;ReadingBooksGenre&gt; { public int Price { get; set; } public DateTime Date { get; set; } public ReadingBooksGenre[] Genres { get; set; } } class TextBook : IBook&lt;TextBooksGenre&gt; { public int Price { get; set; } public DateTime Date { get; set; } public TextBooksGenre[] Genres { get; set; } } enum TextBooksGenre { Math, Science } public enum ReadingBooksGenre { Fiction, NonFiction } } </code></pre>
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