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CHAPTER 3 DEFINING THE FOUNDATION
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In this example, the Factory.Instantiate method has an additional type identifier that would be used to instantiate the correct implementation type. The solution is elegant and rather clever. Consider the following implementation: public class Factory { public static IMathematics Instantiate<type>() { if( typeof( type) is int) { return new ImplIMathematics(); } } } In the implementation of Instantiate, an if block tests which type the Generic .NET parameter is. Then, depending on the type, the appropriate implementation is instantiated. The advantage of this approach is that the Factory class can be extended to support new types without needing the client to be recompiled or changed.
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Controller Interfaces
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In general, the controller class is defined as a class, but sometimes the controller class needs to be defined as an interface. There are several reasons for doing this, and the most important relate to flexibility and convenience. Let s start out by focusing on the flexibility issue. Imagine the controller is implementing a taxation system. The general intention of taxation is similar regardless of the country. What s different are the details of calculating the deductions, amortizations, etc. Some developers might be tempted to abstract the details as parameters that can be activated to determine which calculations are performed by the controller. The controller class would then need to be constantly extended to support yet another taxation system. Such an approach is futile and leads to complicated and messy code that should be avoided. When a controller class needs to switch personality, an interface should be defined for a taxation system such as the following: namespace ITaxation { public interface IIncomes { } public interface IDeductions { } public interface ITaxation { IIncomes [] Incomes { get; set; } IDeductions [] Deductions { get; set; } Decimal CalculateTax(); } } The interface ITaxation has two properties, Incomes and Deductions, and a single method, CalculateTax. The properties are assigned the incomes and deductions of the individual. Instead of properties, you might think another solution would be to convert the properties into parameters for the method CalculateTax; but that would be incorrect, because the taxation system might
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want to perform multiple taxation calculations. Hence, the data is best defined as properties, since the properties are central to the calculation of taxes for the ITaxation interface. As a rule of thumb, if the data is needed in more than two methods, then you should create a property. Let s ignore the implementations of the IIncomes and IDeductions for the time being and consider them as general implementations that will be instantiated and assigned somewhere. Instead, let s focus on the ITaxation interface. Unlike the previous controller definition, there are multiple controller implementations. Each controller implementation would be specific to a country and its tax structure. Following is an example implementation: internal class SwissTaxes : ITaxation { public IIncomes[] Incomes { get { return null; } set { ; } } public IDeductions[] Deductions { get { return null; } set { ; } } public Decimal CalculateTax() { return new Decimal(); } } The class SwissTaxes implements all of the methods. Again, ignore the properties, but consider the method CalculateTax. The method CalculateTax would be a custom implementation of the Swiss tax code. If the class were AmericanTaxes, the method CalculateTax would be an implementation of the American tax code.
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Implementing a Default Base Class
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From the class SwissTaxes just presented, let s consider the properties Incomes and Deductions. In the example class, the properties weren t properly implemented, but this wouldn t be the case in an actual working example. Implementing SwissTaxes means implementing the properties using some type of assignment and retrieval. When coding AmericanTaxes, the same properties have to be implemented, and the exact same source code, or at least very similar, will be used to code American taxes. The problem with an interface is that there is no implementation. When an interface is implemented multiple times, it could occur that multiple implementations are similar if not identical. The solution to the redundancy problem is to include a default base class that implements a base logic. The default base class would then be subclassed by the different controller implementations. An example default base class is as follows: public abstract class BaseTaxation : ITaxation { private IIncomes[] _incomes; private IDeductions[] _deductions;
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