SOLID design principles in .NET: the Open-Closed Principle

Exercises in .NET with Andras Nemes

Introduction

In the previous post we talked about the letter ‘S’ in SOLID, i.e. the Single Responsibility Principle. Now it’s time to move to the letter ‘O’ which stands for the Open-Closed Principle (OCP). OCP states that classes should be open for extension and closed for modification. You should be able to add new features and extend a class without changing its internal behaviour. You can always add new behaviour to a class in the future. At the same time you should not have to recompile your application just to make room for new things. The main goal of the principle is to avoid breaking changes in an existing class as it can introduce bugs and errors in other parts of your application.

How is this even possible? The key to success is identifying the areas in your domain that are likely to change and programming to abstractions. Separate out…

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