A Design and testability in Visual C#.NET

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Appendix A Design and testability
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hanging the design of your code so that it s more easily testable is a controversial issue for some developers. This appendix will cover the basic concepts and techniques for designing for testability. We ll also look at the pros and cons of doing so and when it s appropriate. First, though, let s consider why you would need to design for testability in the first place.
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A.1 Why should I care about testability in my design
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The question is a legitimate one. In designing our software, we re taught to think about what the software should accomplish, and what the results will be for the end user of the system. But tests against our software are yet another type of user. That user has strict demands for our software, but they all stem from one mechanical request: testability. That request can influence the design of our software in various ways, mostly for the better. In a testable design, each logical piece of code (loops, ifs, switches, and so on) should be easy and quick to write a unit test against, one that demonstrates these properties:
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Runs fast Is isolated, meaning it can run independently or as part of a group of tests, and can run before or after any other test Requires no external configuration Provides a consistent pass/fail result
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Design goals for testability
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These are the FICC properties: fast, isolated, configuration-free, and consistent. If it s hard to write such a test, or it takes a long time to write it, the system isn t testable. If you think of tests as a user of your system, designing for testability becomes a way of thinking. If you were doing test-driven development, you d have no choice but to write a testable system, because in TDD the tests come first and largely determine the API design of the system, forcing it to be something that the tests can work with. Now that you know what a testable design is, let s look at what it entails, go over the pros and cons of such design decisions, and discuss alternatives to the testable design approach.
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A.2 Design goals for testability
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There are several design points that make code much more testable. Robert C. Martin has a nice list of design goals for object-oriented systems that largely form the basis for the designs shown in this chapter. See his Principles of OOD article at http://butunclebob.com/ArticleS.UncleBob.PrinciplesOfOod. Most of the advice I include here is about allowing your code to have seams places where you can inject other code or replace behavior without changing the original class. (Seams are often talked about in connection with the Open Closed Principle, which is mentioned in the Martin s Principles of OOD article.) For example, in a method that calls a web service, the web service API can hide behind a web service interface, allowing us to replace the real web service with a stub that will return whatever values we want, or with a mock object. s 3 5 discuss fakes, mocks, and stubs in detail. Table A.1 lists some basic design guidelines and their benefits. The following sections will discuss them in more detail.
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APPENDIX A
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Design and testability
Table A.1 Test design guidelines and benefits
Design guideline Make methods virtual by default. Benefit(s) This allows you to override the methods in a derived class for testing. Overriding allows for changing behavior or breaking a call to an external dependency. This allows you to use polymorphism to replace dependencies in the system with your own stubs or mocks. You can t override anything virtual if the class is sealed (final in Java). This allows you to serve up your own fake instances of classes to methods that require them, instead of being tied down to working with an internal production instance of a class.
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