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public class SwissTaxMathFactory { public static TaxMath Instantiate() { return new SwissTaxMath(); } } The class SwissTaxMath derives from TaxMath. The factory SwissTaxMathFactory instantiates the type SwissTaxMath and returns the type TaxMath. When using classes as interfaces, don t ignore the use of the keyword abstract. In the previous examples, it wasn t necessary, but it avoids the problem of having a user instantiate the type directly. For example, to avoid instantiating the TaxMath class directly, the following code can be used: public abstract class TaxMath { public virtual Decimal IncomeTax( Decimal rate, Decimal value) { return new Decimal(); } } internal class StubTaxMath : TaxMath { } public class TaxMathFactory { public static TaxMath Instantiate() { return new StubTaxMath(); } } In the example, the class StubTaxMath derives from the abstract-scoped class TaxMath. The scope of the class StubTaxMath is internal, so it can t be inappropriately instantiated. The factory TaxMathFactory instantiates the type StubTaxMath, but returns the abstract class TaxMath type. Using classes and abstract classes instead of interfaces is useful when the implementations are narrowly scoped and tend to be reusable. For example, nobody would ever think of defining strings as an interface. The string type solves a narrowly scoped problem and is often used. Interfaces define reusable contracts that are implemented in different contexts, and they solve a narrowly defined problem. If an interface tries to solve too many problems, the resulting implementations become unwieldy and problematic. An implementation of an interface is considered a modular solution and not reusable because it solves a single problem. For example, the Swiss or American taxation implementations aren t reusable because they solve a narrowly scoped problem. The Swiss and American taxation implementations are modular because they are used in the overall application to calculate taxations for various countries. When using classes as interfaces, the classes are considered reusable and not modular. For example, a tax calculator is used in multiple contexts such as for the American, Canadian, German, and British taxation systems. But when defining a reusable class, often it s necessary to specialize the functionality, and as such keywords like virtual or abstract need to be used. Now that you ve become familiar with the Bridge pattern and it s variations, I want to turn your attention next to instantiating types with the Factory pattern.
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Instantiating Types with the Factory Pattern
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The Factory pattern2 is used to instantiate a type. The simplest of factories is one that has a single method and instantiates a single type. Such a factory doesn t solve all problems in all contexts, and therefore different instantiating strategies have to be employed. All of these strategies are creational patterns that operate similarly to the factory. Specifically, in this section we ll explore why you want to use helper types, as well as how to create plug-ins, how to implement objects according to a plan, and when to clone objects.
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The helper type used to instantiate another type has been illustrated in multiple places thus far in the chapter. Also outlined were some reasons why using helper objects is a good idea. What wasn t covered are the detailed reasons why it s a good idea. Helper objects make it simpler to keep the details of instantiating a type hidden from the consumer. Let s say that a type needs to be instantiated. The consumer uses the new keyword to instantiate a type that implements an interface. Remember that the implementation of an interface is modular. It could be that one implementation operates under one set of conditions, and another uses a different set of operating conditions. The simplest example is that one implementation needs to be used as a singleton, and another can be instantiated for each method call. (A singleton is a single instance of a type.) Consider the following code that illustrates different operating conditions: public interface SimpleInterface { } internal class MultipleInstances : SimpleInterface { } internal class SingleInstance : SimpleInterface { } public class SimpleInterfaceFactory { public static SimpleInterface FirstType() { return new MultipleInstances(); } private static SingleInstance _instance; public static SimpleInterface SecondType() { if( _instance == null) { _instance = new SingleInstance(); } return _instance; } } The interface SimpleInterface is considered a reusable type that is implemented by the classes MultipleInstances and SingleInstance. The difference with the class SingleInstance is that there can only ever exist a single instance. The factory class SimpleInterfaceFactory has
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2. Design Patterns: Elements of Reusable Object-Oriented Software, pp. 87, 107.
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