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Chapter 5. Interfaces and Contracts

Chapter 5. Interfaces and Contracts

During your years as a software developer, you've likely come across the notion of interface-based programming. If you're familiar with the seminal book, Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (known as the "Gang of Four"),[] then you know that many design patterns employ interface-style "contracts." If you're not familiar with that book and its concepts, I urge you to read it. In this chapter, it is my goal to show you how you can model well-defined, versioned contracts using interfaces. In this context, a contract is an agreement by a type to support a set of functionality.

[] Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (Boston, MA: Addison-Wesley Professional, 1995) is cited in the references at the end of this book.

If you've done any COM or CORBA development over the years, then you've most definitely been doing interface-based development. In fact, the interface is the only form of communication between components in COM. Therefore, much of the design complexity rests in developing solid interfaces before you write any line of implementation code. Failure to follow this paradigm has been the source of many problems. For example, Visual Studio 2003 offered an easy environment from which you could create web services. By simply annotating methods of a class a certain way, you could expose those methods as methods of the web service. However, the IDE fostered an approach whereby the interface was the result of annotating methods on a class rather than the other way around. Thus, the cart was put before the horse. Instead, you should clearly define the web service interface before doing any coding, and then code the implementation to implement the interface. To name just one benefit of this approach, you can code both the client and the server concurrently rather than one after the other. Another part of the problem is that once an interface is published to the world, you cannot change it. Doing so would break all implementations based upon it. Unfortunately, the Visual Studio environment encourages you to break this rule by making it easy for you to add a new method to a class and annotate it as a web service method.


  

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