How do you apply the open-closed principle in your software design process?

In my experience Software Design process is recursive and iterative in its own nature, While designing the entities, the solid priciples should be kept in mind, which goes hand in hand with Object oriented programming principles. While open/close says an entity is open for extension and close for modification, the original entity should never be modified, as those changes would have to be modified across all reference and would violate Single responsibility (in most cases). Adding an extension class (for some cases child class)/interface would follow open close and would make the design intact.

?? Abstractions and polymorphism are used to implement the Open-Closed Principle (OCP). Abstraction hides implementation details behind interfaces or abstract classes, whereas polymorphism allows different subclasses to share the same interface or abstract class. As a result, we can extend functionality without altering existing code by adding new implementations. Polymorphism allows us to treat all shapes uniformly by implementing an interface Shape with a draw method.

Incorporating the open-closed principle, which dictates that software entities should be open for extension but closed for modification, has been a cornerstone of my design process. A vivid example is designing a payment processing system. By using abstraction to define a common interface for payment methods and polymorphism to extend this interface for specific payment types (like credit cards, PayPal, etc.), we create a system that can easily accommodate new payment methods without altering the existing codebase. This approach ensures that adding or changing payment types becomes a matter of extending the system rather than modifying existing functionalities, thereby adhering to the open-closed principle.

Create libraries (aka, DLLs) and try to extend the libraries from the final application, without doing any changes to the existing libraries. If you need to change the libraries because of application requirements, that means the libraries aren't following OCP correctly. That is, assuming the library is finished, it should not change by application specific requirements. If needed, the application should be able to "extend" it, usually by implementing some interface or abstract class and maybe registering it in a manager component that exists in the library. This can be achieved by using dictionaries, be it a dictionary of file extension to image loader, text extension to text loader, class type to object serializer etc.

When implementing the open-closed principle (OCP) in software design, I lean heavily on the power of abstraction and polymorphism. ? Abstraction centralizes shared properties in a class hierarchy, encapsulating common functionality. ? Polymorphism allows code to operate on the base abstract class or interface, deferring the exact behavior to subclasses. This structure enables systems to grow with new classes that adhere to established contracts without changing existing code. For example, in a payment processing system, an abstract class 'Payment' defines a 'process' method, while subclasses 'CreditCard' and 'PayPalPayment' provide specific implementations, allowing seamless integration of future payment methods.

Dependency inversion is one of the efficient way for following open/close principle, rather than employing the concrete entities which might then break Single Responsibility Principle, or extending/inherting to child entity would leave the dependency in parent class for the implementation, however with dependency inversion the core implementation would be in concrete class which in turn would have nothing to do with implementation entity, leaving the dependency to conrete class. This could be achieved by dependency injection.

?? Use dependency inversion by having software entities depend on abstractions instead of concrete implementations to implement the Open-Closed Principle (OCP). Injecting these implementations at runtime allows you to change message destinations without altering the Logger class and reduces module coupling. For example, a Logger class should depend on a Writer interface, which FileWriter, DatabaseWriter, or ConsoleWriter can implement.

DIP dictates following **three rules.** 1. DIP dictates that **high-level modules should not depend on low-level modules.** - **high-level class should not call or instantiate low-level class directly. - Will see in code below** 2. **Both high-level modules and low-level modules should depend on abstractions.** - High level class should use low-level class functionality via interface. - Will see this in action in code below 3. **Furthermore, abstractions should not depend on details; details should depend on abstractions.** - you create interface (called abstraction) which low-level class will implement.

Dependency inversion, a key aspect of the SOLID principles, plays a crucial role in applying the open-closed principle by decoupling modules. For instance, consider a reporting module in a software application. Initially, it might generate reports only in PDF format. By employing dependency inversion, the module can rely on an abstract report generator rather than concrete implementations. Later, if the need arises to support Excel reports, a new Excel report generator can be introduced and injected without altering the reporting module's code. This not only keeps the module open for extension (supporting new report formats) but also closed for modifications, eliminating the need for any changes to the module itself for new formats.

Applying dependency inversion enhances modularity and eases maintenance. To exemplify, in managing a logging module, I prefer implementing a ILogDestination interface, which standardizes the output method across various destinations. By developing separate classes like FileLogDestination and ErrorReportingLogDestination conforming to this interface, I can effortlessly switch or extend logging targets without modifying the core logging logic. It's pivotal to design these abstractions judiciously to truly realise the open-closed principle's benefits. This methodology fosters a plug-and-play ecosystem within the software, fostering a resilient and adaptable codebase.

?? Applying the Open-Closed Principle (OCP) requires leveraging design patterns that provide reusable solutions and adhering to good design principles. The Strategy, Template Method, and Decorator patterns are effective: Strategy encapsulates algorithms in separate classes with a common interface, Template Method defines algorithm skeletons in abstract classes, and Decorator adds new behavior by wrapping objects dynamically. We can extend functionality without modifying existing classes by using these patterns.

OCP states that we should be able to add a new functionality without modifying the existing functionality, obviously except when there is a bug which needs to be fixed. Some additional guidelines on how to apply the Open-Closed Principle in your software design process are: - Create stable base classes which are unlikely to change frequently. This helps in making sure that the classes are "closed". - Modular design: Break the solution in set of independent modules or components that can be extended without modifying each other. Each module should have a well defined responsibility. Any new feature should be implemented as a new module. -Comprehensive Unit test: Helps to make sure that we are not modifying the existing functionality.

In my experience using design patterns we can enforce Open Close Principle. For example: One time at a project I was working on, we have developed workflow which was made in State Pattern, which only had implementation of changing of state and the business logic to perform on change of state is done following a repository pattern, which helped us perform seggregation of concerns, such that the common workflow can be added for different type of operations increasing reusability of code and enforcing open close principle, that's how we have enforced open close by following design patterns

Design patterns, like the Strategy pattern, perfectly embody the open-closed principle in my work. For instance, in a logistics system, we used it to switch routing algorithms based on various criteria without altering the core code. This pattern allowed us to encapsulate different algorithms behind a unified interface, making the system adaptable to new requirements while keeping the original code unchanged. Such practices highlight design patterns' role in enhancing software flexibility and maintainability.

In harnessing design patterns to uphold the open-closed principle, I often lean on the Strategy pattern for its versatility. By segregating strategy algorithms into distinct classes, I can ensure they fulfil a common interface, thus enabling effortless substitution and extension without impacting existing code. This approach has been quintessential in applications where multiple interchangeable behaviours are necessary, offering the flexibility and scalability needed for resilient software architecture. It champions modularity and adheres to the tenet of "open for extension, closed for modification."