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CHAPTER 3 DEFINING THE FOUNDATION
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Keeping Options Open Using Interfaces
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When defining interfaces, the objective should be to solve the task at hand, no more, no less. Many developers attempt to future-proof interfaces by adding features that could potentially be used in the future. The problem with adding such features is that they need to be tested and documented. The reality is features will be added, not properly tested, not properly used, and become dead code that has to be maintained in the future. By keeping interfaces to the minimal requirements, it s possible to solve the problem without getting bogged down in extra work. For example, from 2, recall that the type Mathematics was defined as well as a single method, Add, that calculated everything using the type int. A good way to future-proof the type would be to not use the type int, but use some general type. An example of using a general type is to employ Generics, as illustrated here: public interface IMathematics< numbertype> { numbertype Add( numbertype param1, numbertype param2); } The interface IMathematics is defined using a Generic data type, which allows you to define the specific data type at a later point in time. An implementation of the IMathematics interface is as follows: internal class IntMathematicsImpl : IMathematics< int> { public int Add(int param1, int param2) { checked { return param1 + param2; } } } The class IntMathematicsImpl implements the interface IMathematics and defines the data type to be an int. Then, like the original implementation of Mathematics, the two integers are added in the context of checked block. A factory that instantiates the class IntMathematicsImpl is defined as follows: public class FactoryIMathematics { public static IMathematics< int> Instantiate() { return new IntMathematicsImpl(); } } The method Instantiate returns a specialized form of the IMathematics interface that uses the int data type. The code within the method Instantiate instantiates a new instance of the class IntMathematicsImpl.
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The interface that used Generics to keep options open allows different types to be manipulated. A developer might come up with the idea to convert the implementation and factory to use Generics, thus making it possible to manipulate all types without having to explicitly provide
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CHAPTER 3 DEFINING THE FOUNDATION
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an implementation for the type. The following example illustrates how the factory could be converted into a Generic type: public class FactoryIMathematics< basetype> { public static IMathematics< basetype> Instantiate() { return new IntMathematicsImpl< basetype>(); } } The class FactoryIMathematics is converted into a Generic type, and the method Instantiate uses the Generic type, as does the class IntMathematicsImpl. Following is the Generic type IntMathematicsImpl: internal class IntMathematicsImpl< basetype> : IMathematics< basetype> { public basetype Add( basetype param1, basetype param2) { checked { return param1 + param2; } } } The converted IntMathematicsImpl class is based entirely on using a Generic type. In theory, it s possible to use any data type, and through the magic of Generics, it s possible to perform add operations on any type. The problem is that it isn t possible to magically add any data type. This is an example of abusing Generic types. The class IntMathematicsImpl won t compile because C# has no idea how to translate the expression param1 + param2, as the compiler doesn t understand how to add two unknown types together. This means it isn t possible to use Generics to add two unknown types. A developer might then come up with the idea of abstracting the Add method to use another interface that a type must implement la .NET constraints. Using constraints, the Generic implementation knows how to add two unknown data types. This solution would work, but it s a delegation, which adds complexity. Remember, the original interface generically defined how to perform mathematics in a general fashion. The added complexity in the example might be obvious, but it happens because one idea leads to another idea, which leads to yet another idea. It s very easy to overuse Generics, as it s so tempting to define everything in abstract terms. You might want to think that, once those abstract terms have been defined, magically plugging in a data type will result in a working system. The problem is that it s impossible to do that. Adding methods or properties to make the Generic types work is adding complexity. Remember the focus of the Bridge pattern: to separate intention from implementation. An intention can and needs to be described in abstract terms, but an implementation should be specific. Another danger of attempting to define in abstract terms an addition is that adding integers isn t like adding complex numbers, floating-point numbers, or even different bases. The result is that the different types share the same intention of adding, but their implementations will be entirely different.
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