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Managing Extensions Using Polymorphism
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When a single class implements a single interface, the code is easy to understand and manage. The client that consumes the interface doesn t have to worry about anything, because the implemented functionality is contained within the interface. Now imagine the situation when two different versions of an interface have to be consumed. Does that mean a client has to instantiate two different interface instances Or how about the situation when a client needs to access multiple functionalities Does that require multiple interface instances Polymorphism is the ability to process objects differently depending on the context. The Extension pattern8 makes it possible to define polymorphism in multiple forms and enables you to add functionality without having to necessarily modify the already existing functionality. You might be thinking, But hey, what about the Decorator pattern The answer is that the Extension pattern doesn t preclude the Decorator pattern, the Strategy pattern, the State pattern, or any other pattern. What the Extension pattern does is provide a mechanism to implement the Decorator pattern. The Extension pattern comprises two variations, the Static Extension pattern and the Dynamic Extension pattern, both of which we ll look at in the following text.
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Implementing the Static Extension Pattern
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The simplest version of the Extension pattern is the Static Extension pattern. The Static Extension pattern is implemented using interfaces. This has the benefit of being able to extend the functionality of an implementation by attaching multiple interfaces as illustrated by the following example: public interface IBase1 { } public interface IBase2 { } public class Implementation : IBase1, IBase2 { } The class Implementation has implemented the interfaces IBase1 and IBase2. Granted the interfaces have no methods, but the overall idea is illustrated. For example, if IBase1 is the original, then when Implementation implements IBase2, the implementation of IBase1 doesn t need to be modified. In UML terms, the Static Extension pattern is identical to what you see in Figure 6-4.
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8. James O. Coplien and Douglas C. Schmidt, Pattern Languages of Program Design (Boston: AddisonWesley, 1995), p. 81.
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Figure 6-4. UML diagram that illustrates two interfaces implemented by a single class The Static Extension pattern is a fundamental part of the C# programming language, and you probably haven t even realized that you were using it. The following test code illustrates how an interface can be tested for existence: [TestFixture] public class TestExtensions { [Test] public void TestStaticExtension() { IBase1 extension = new Implementation (); Assert.IsTrue( extension is IBase2); } } The test code method TestStaticExtension instantiates the class Implementation and assigns it to a variable of type IBase1. The Static Extension pattern occurs when Assert is tested for the existence of the IBase2 interface, even though the variable references the type IBase1. The Assert.IsTrue method doesn t fail and indicates that indeed the variable extension can be converted to the type IBase2. What is happening is that the .NET runtime is capable of doing a conversion to IBase2 because the implementation of IBase1 is the class Implementation, which also implements IBase2.
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Associating a Single Method to Multiple Interfaces
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Now let s complicate the scenario by having both IBase1 and IBase2 define the same method: public interface IBase1 { int Value(); } public interface IBase2 { int Value(); }
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Both interfaces define the method Value, which has an int return value. The simplest way to implement the interfaces is illustrated by the following class declaration: public class ImplementationBoth : IBase1, IBase2 { public int Value() { return 1; } } ImplementationBoth implements both IBase1 and IBase2, and the method implemented by both interfaces is Value. The following test code illustrates that when IBase1.Value and IBase2.Value are called, the value of 1 is returned: [TestFixture] public class TestExtensions { [Test] public void TestStaticExtension() { IBase1 base1 = new ImplementationBoth(); IBase2 base2 = base1 as IBase2; Assert.AreEqual( 1, base1.Value()); Assert.AreEqual( 1, base2.Value()); } } The method TestStaticExtension instantiates the type ImplementationBoth and is assigned to base1, which is the type IBase1. Then the variable base2 is assigned using a typecast defined by the statement base1 as IBase2. The Assert tests illustrate that regardless of whether the Value method is called from IBase1 or IBase2, the same value of 1 is returned. If you aren t familiar with the as statement, note that it doesn t generate an exception if the interface can t be cast. If the interface can t be cast to the type, then the variable will have a value of null. To generate a cast exception, the cast (IBase2)base would have to be used. Let s step back and consider what has happened. Imagine a class has to implement a new interface in your application. The new interface has some methods that are identical to the old interface and some methods that are unique to the new interface. For those methods that are identical to the old interface, the same method will be called.
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