Visual Basic 2005 with .NET 3.0 Programmer's Reference

A class’s public interface is the set of properties, methods, and events that are visible to code outside of the class. The class may also have private properties, methods, and events that it uses to do its job. For example, the Job class described in the previous section provides an AssignJob method. That method might call a private FindQualifiedEmployee function, which looks through an employee database to find someone who has the skills and equipment necessary to do the job. That routine is not used outside of the class, so it can be declared private.

The class may also include properties and events hidden from code outside of the class. These hidden properties, methods, and events are not part of the class’s public interface.

The class encapsulates the programming abstraction that it represents (a Job in this ongoing example). Its public interface determines what is visible to the application outside of the class. It hides the ugly details of the class’s implementation from the rest of the world. Because the class hides its internals in this way, encapsulation is also sometimes called information hiding.

By hiding its internals from the outside world, a class prevents exterior code from messing around with those internals. It reduces the dependencies between different parts of the application, allowing only those dependencies that are explicitly permitted by its public interface.

Removing dependencies between different pieces of code makes the code easier to modify and maintain. If you must change the way the Job class assigns a job to an employee, you can modify the AssignJob method appropriately. The code that calls the AssignJob routine doesn’t need to know that the details have changed. It simply continues to call the method and leaves the details up to the Job class.

Removing dependencies also helps break the application into smaller, more manageable pieces. A developer who calls the AssignJob method can concentrate on the job at hand, rather than on how the routine works. This makes developers more productive and less likely to make mistakes while modifying the encapsulated code.

The simpler and cleaner a class’s public interface is, the easier it is to use. You should try to hide as much information and behavior inside a class as possible while still allowing the rest of the program to do its job. Keep properties, methods, and events as simple and focused as possible. When you write code that the class needs to use to perform its tasks, do not expose that code to the outside program unless it is really necessary. Adding extra features complicates the class’s public interface and makes the programmer’s job more difficult.

This can be a troublesome concept for beginning programmers. Exposing more features for developers to use gives them more power, so you might think it would make their jobs easier. Actually, it makes development more difficult. Rather than thinking in terms of giving the developer more power, you should think about giving the developer more things to worry about and more ways to make mistakes. Ideally, you should not expose any more features than the developer will actually need.

Note that you can achieve many of the benefits of encapsulation without classes. Decades before the invention of object-oriented languages, programmers were making code libraries that encapsulated functionality. For example, a library of trigonometry functions would expose public function calls to calculate sines, cosines, tangents, arctangents, and so forth. To perform its calculations, the library might contain private lookup tables and helper functions that calculate series expansions. The tables and helper functions were hidden from the main program calling the library functions.

One big benefit of classes over this sort of library encapsulation is intuitiveness. When you make a class named Job, Customer, or Employee, anyone familiar with your business can make a lot of assumptions about what the class represents. Even if you don’t know how the Job class works, you can probably guess what a new ReassignJob method would do. As long as everyone has a common vision of what the class does, you get an extra level of intuitive encapsulation that you don’t necessarily get with a more procedural approach.