15.2. Csproject Revisited

The cs_project1 project skeleton was set up with the different commands in different classes, keeping related things together.

On the other hand they had high level structure in common. Similar names were consciously used for methods:

  • Each command needed to Execute
  • Each command needed to have Help for the user.

The corresponding names made it somewhat easier to follow the part of the Game constructor with the additions to the helpDetails Dictionary. Also there is repetitive logic in the crucial proccessCommand method.

In a game with more possible commands, the code would only get more repetitious! You would like to think of having a loop to replace the repetitious code.

A major use of a C# interface will allow this all to work in neat loops. For the first time we define our own interface, and use that interface as a type in a declaration.

While we are at this we can refactor our code further: classes that give a response to a command all obviously have their Execute and Help methods. They also have a command word to call them. We can further encapsulate all data for the response by having the classes themselves be able to announce the command that calls them. We add a string property CommandWord to each of them.

We will add an extra convenience feature of C# here. Thus far we have used private instance variables and public getter methods. We can use a public instance variable declaration with a similar effect as in:

public string CommandName {get; private set;}

The extra syntax in braces says that users in a another class may freely get (read) the variable, but setting the variable is still private: it may only be done inside the class. This is more concise than using a getter method: No getter needs to be declared; referencing the data is shorter too, since it is a property, no method parentheses are needed.

Note the unusual syntax: the declaration does not end with a semicolon. The only semicolons are inside the braces. You will not be required to code with this notation, but it surely is neater than using a getter method!

Now we can define our own interface taking all of these common features together. Since each is a response to a command, we will call our interface Response:

namespace IntroCS
/// Object that responds to a command
   public interface Response
      /// Execute cmd.
      /// Return true if the game is over; false otherwise
      bool Execute(Command cmd);

      /// Return a Help string for the command
      string Help();

      string CommandName {

Things to note:

  • The heading has the reserved word interface instead of class.
  • All the common method headings and the property declaration are listed.
  • See what is missing! In place of each method body is just a semicolon.
  • Everything in an interface is public. The part of the property about private access is merely omitted.

We are going to need a collection if we want to simplify the code with loops. We could use code like the following, assuming we already declared the objects helper, goer, and quitter:

Response[] resp = {helper, goer, quitter};

See how we use Response as a declaration type! Each of the objects in the declaration list is in fact a Response.

Now that we can process with this collection and foreach loops, we do not need the object names we gave at all: We can just put new objects in the initialization sequence!

Now that we can think of these different objects as being of the same type, we can see the processCommand logic, with its repetitive if statement syntax is just trying to match a command word with the proper Response, so a Dictionary is what makes sense!

In fact all the logic for combining the various Responses is now moved into CommandMapper, and the the CommandMapper constructor creates the Dictionary used to look up the Response that goes with each command word. Here is the whole code for ResponseMapper, taking advantage of the Dictionary in other methods, too.

using System;
using System.Collections.Generic;
namespace IntroCS

/// Map commands names to commands.
   public class CommandMapper
      public string AllCommands {
         private set;
      private Dictionary<string, Response> responses; //responses to commands

      /// Initialize the command response mapping
      ///  game The game being played.
      public CommandMapper(Game game)
         responses = new Dictionary<string, Response>();
         Response[] resp = {
            new Quitter(),
            new Goer(game),
            new Helper(responses, this)
            // add new Responses here!
         AllCommands = "";
         foreach (Response r in resp) {
            responses[r.CommandName] = r;
            AllCommands += r.CommandName + " ";

      /// Check whether aString is a valid command word.
      /// Return true if it is, false if it isn't.
      public bool isCommand(string aString)
         return responses.ContainsKey(aString);

      /// Return the command associated with a command word.
      ///  cmdWord The command word.
      /// Return the Response for the command.
      public Response getResponse(string cmdWord)
         return responses[cmdWord];

There is even more to recommend this setup: The old setup had references in multiple places to various details about the collection of Responses. That made it harder to follow and definitely harder to update if you want to add a new command. Now after writing the new class to respond to a new command, the only thing you need to do is add a new instance of that class to the array initializer in the CommandMapper constructor!


Interfaces never say anything about constructors. Classes satisfying an interface can have totally different constructors.

In the code above, Quitter has a very simple constructor. (A Quitter has no individualized data, and only has an instance because an interface can only work with an instance, not with a static class.) On the other hand the Goer and Helper objects need to reference more data to work properly, so they have parameters.

The revised Xamarin Studio project is csproject_stub (no 1 this time).

See how the Game class is simplified, too.

Talking about adding commands - these classes could be the basis of a game project for a small group. Have any ideas? See Group Project. Note that there is a data file in the project directory. Make sure your project, like this stub, sets the Output Path to the project folder.

There are further examples of defining and using Interfaces in the starting code for the exercises at the end of the next section.

15.2.1. Cohesion, Coupling, and Separation of Concerns

This section is motivated by the revisions to the project, not specifically about Interfaces, though good use of them helps.

There are three important ideas in organizing your code into classes and methods:

Cohesion of code is how focused a portion of code is on a unified purpose. This applies to both individual methods and to a class. The higher the cohesion, the easier it is for you to understand a method or a class. Also a cohesive method is easy to reuse in combination with other cohesive methods. A method that does several things is not useful to you if you only want to do one of the things later.

Separation of concerns allows most things related to a class to take place in the class where they are easy to track, separated from other classes. Data most used by one class should probably reside in that class. Cohesion is related to separation of concerns: The data and methods most used by one class should probably reside in that class, so you do not need to go looking elsewhere for important parts. Also, if you need to change something, where concerns are separated into cohesive classes, with the data and logic in one place, it is easier to see what to change, and the changes are likely to be able to be kept internal to the class, affecting only its internal implementation, not its public interface.

Some methods are totally related to the connection between classes, and there may not be a clear candidate for a class to maximize the separation of concerns. One thing to look at is the number of references to different classes. It is likely that the most referred to class is the one where the method should reside.

Coupling is the connections between classes. If there were no connections to a class, no public interface, it would be useless except all by itself. The must be some coupling between classes, where one class uses another, but with increased cohesion and strong separation of concerns you are likely to be able to have looser coupling. Limiting coupling makes it easier to follow your code. There is less jumping around. More important, it is easier to modify the code. There will be less interfacing between classes, so if you need to change the public interface of a class, there are fewer places in other classes that need to be changed to keep in sync.

Aim for strong cohesion, clear separation of concerns, and loose coupling. Together they make your code clearer, easier to modify, and easier to debug. IGame Interface Exercise

On a much smaller scale than the project, this exercise offers you experience writing classes implementing and using an interface.

  1. Copy project stub igame_stub to your own project, and modify it as discussed below.

  2. Look at the IGame interface in i_game.cs. Then look at addition_game.cs, that implements the interface. See how a new AdditionGame can be added to list of IGame‘s. Run play_games.cs. Randomly choosing a game when there is only one to choose from is pretty silly, but it gives you a start on a more elaborate list of games. The PopRandom method is a good general model for choosing, removing, and returning a random element.

  3. Write several very simple classes implementing the IGame interface, and modify Main in play_games.cs to create and add a new game of each type. (Test adding one at a time.)

    One such game to create with little more work would be a variation on instance based Guessing Game instance_version/guess_game.cs. You need to make slight modifications. You could make Play return the opposite of the number of guesses, so more guesses does generate a worse score. Note that you could not use the original static game version: Only objects can satisfy an interface. Bisection With Function Interface Exercise

See bisection_method/bisection_method.cs. Identify the interface. See how two new classes satisfy the interface, while the rest of the code in these classes is quite different: One has an instance variable and an explicit constructor, while the other does not. See that the Bisection function now has a parameter for an object containing the mathematical function to use for root finding: Bisection is now a generally useful method, not just tied to one hard-coded function that might have a root.

Add a new class satisfying the Function interface; add a test in Main. Try a function with multiple roots in the original interval and see what happens. Then, using distinct intervals in different tests, find different roots of the same function.