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Refactoring Switch Statements (Take 2)

, 2 Jul 2009
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How to refactor Switch statements.

Hello to everyone, had been such a long time since my last article. Many interesting things had been going on in Sansir lately that had been taking my entire free time, and also I’m a man with strong family links, so that makes it difficult to publish articles frequently. I am not complaining about it, not at all, color me “happy”.

OK, stop whining.

In one of our previous articles, I showed you how to refactor a piece of code which was deciding what method to execute based on the evaluation result of a switch statement; you can read about it here.

As a final note, I stated in the article that the presented code could be refactored even further, because if you look carefully, we had just moved the maintenance nightmare to another place in the program, but the original problem still exists.

What to do then? This is what I would do:

  1. Create a specific class for each sport that is going to be handled; those classes must implement a simple interface and they need to be decorated with an attribute where is specified the target sport that it will handle. Like:
  2. public interface INewsDisplayer
    {
        void Display();
    }
    
    public class NewsDisplayerDefinitionAttribute : Attribute 
    { 
        public NewsDisplayerDefinitionAttribute(Sports sport) 
        { 
            Sport = sport; 
        } 
        public Sports Sport { get; private set; } 
    }

    An implementation of this would look like:

    [NewsDisplayerDefinition(Sports.Soccer)]
    public class SoccerNewsDisplayer : INewsDisplayer 
    { 
    #region INewsDisplayer Members 
        public void Display() 
        { 
            Console.WriteLine("Displaying News for Soccer"); 
            // Real implementation below 
            // Do something 
        } 
    #endregion 
    }
  3. Using the Factory Pattern, create a class that takes a Sport parameter and returns an IEnumerable<INewsDisplayer> instance. One difference with our previous attempt is that we are going to be able to define more than one implementation for a given Sport. This factory class will scan all the assemblies for the current AppDomain, and if it finds a type which follows the pattern that I have described in the previous point, it will read the metadata and append an instance of this type to a dictionary that will serve us as our lookup table to return the correct instances for the requested Sport. This factory class looks like:
  4. public static class NewsDisplayerFactory 
    { 
        private static readonly IDictionary<Sports, IEnumerable<INewsDisplayer>> 
          lookupTable = new Dictionary<Sports, IEnumerable<INewsDisplayer>>(); 
        static NewsDisplayerFactory() 
        { 
            BootStrap(); 
        }
    
        private static void BootStrap() 
        { 
            // Find all types in all the assemblies from the current appdomain 
            // that have a correct implementation of 
            // News Displayer (NewsDisplayerDefinitionAttribute + INewsDisplayer) 
            var interfaceFullName = typeof (INewsDisplayer).FullName; 
            var newsDisplayerAttributeType = typeof (NewsDisplayerDefinitionAttribute); 
            var result = from 
                assembly in AppDomain.CurrentDomain.GetAssemblies() 
                from 
                type in assembly.GetTypes() 
                let 
                definition = type 
                .GetCustomAttributes(newsDisplayerAttributeType, true) 
                .Cast<NewsDisplayerDefinitionAttribute>() 
                .FirstOrDefault() 
                where 
                type.GetInterface(interfaceFullName) != null && definition != null 
                group 
                (INewsDisplayer) Activator.CreateInstance(type) 
                by definition.Sport; 
    
            // Filling the dictionary 
            foreach (var item in result) 
            { 
                lookupTable.Add(item.Key, item.ToList()); 
            } 
        } 
    
        public static IEnumerable<INewsDisplayer> GetInstance(Sports sport) 
        { 
            IEnumerable<INewsDisplayer> newsDisplayer; 
            if (lookupTable.TryGetValue(sport, out newsDisplayer)) 
            { 
                return newsDisplayer; 
            } 
            else 
            { 
                throw new NotImplementedException(string.Format(
                  "The method for the sport {0} is not implemented", sport)); 
            } 
        } 
    }
  5. Using the Strategy Pattern, we are going to change our SportNews class to change its context by passing to it the kind of Sport that it is going to handle. That will look like:
  6. public class SportNews 
    { 
        private IEnumerable<INewsDisplayer> newsDisplayer; 
    
        public void SetContext(Sports sport) 
        { 
            newsDisplayer = NewsDisplayerFactory.GetInstance(sport); 
        } 
    
        public void DisplayNews() 
        { 
            foreach (var displayer in newsDisplayer) 
            { 
                displayer.Display(); 
            } 
        } 
    }

    That’s it, with our new approach, we can continue adding NewsDisplayers and use those in our program without touching other parts of our program; doing it by adding more classes to our assembly or dropping assemblies into the bin directory (isn’t this called the plug-in model?).

Finally, I’ll like to show our Program class where every component is used to perform the original idea for this program:

class Program 
{ 
    private static string options = null; 
    private static readonly Type sportsEnumType = typeof(Sports); 
    static void Main(string[] args) 
    { 
        var news = new SportNews(); 
        while (true) 
        { 
            Console.WriteLine(); 
            DisplayOptions(); 
            var key = Console.ReadKey().KeyChar.ToString(); 
            Console.WriteLine(); 
            try 
            { 
                var sport = (Sports)(Enum.Parse(sportsEnumType, key)); 
                news.SetContext(sport); 
                news.DisplayNews(); 
            } 
            catch (Exception ex) 
            { 
                Console.WriteLine(ex); 
                Console.ReadLine(); 
                return; 
            } 
        } 
    } 
    private static void DisplayOptions() 
    { 
        if (options == null) 
        { 
            StringBuilder optionsBuilder = new StringBuilder(); 
            FieldInfo[] enumFields = sportsEnumType.UnderlyingSystemType.GetFields(
                                     BindingFlags.Public | BindingFlags.Static); 
            foreach (FieldInfo enumField in enumFields) 
            { 
                object enumValue = enumField.GetRawConstantValue(); 
                Sports sport = (Sports)(enumValue); 
                optionsBuilder.AppendFormat("To display the news for {0} press {1}\n", 
                                            sport, enumValue); 
            } 
            options = optionsBuilder.ToString(); 
        } 
        Console.WriteLine(options); 
    } 
}

You can download the working sample from here.

Happy coding, see you soon.

Shameless plug: You can check this article on my blog here

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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emiaj
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Peru Peru
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Comments and Discussions

 
GeneralMy vote of 5 PinmemberAnonymous delegate28-Sep-10 6:44 
Actually a very ellegant solution for complex case statements!
GeneralMy vote of 1 PinmemberPavel Vladov8-Jul-09 2:04 
GeneralRe: My vote of 1 Pinmemberemiaj10-Jul-09 5:21 

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