What is SOLID principal?

 SOLID is an acronym that stands for five principles of object-oriented programming design: Single Responsibility Principle, Open-Closed Principle, Liskov Substitution Principle, Interface Segregation Principle, and Dependency Inversion Principle. These principles help in designing software that is easy to maintain, understand, and extend.

Let's take an example in C# to understand SOLID principles. Suppose we have a class called `Car` that represents a car object. Here's how each SOLID principle can be applied:

1. Single Responsibility Principle (SRP): The `Car` class should have only one reason to change. It should be responsible for managing the car's properties and behavior related to a car, such as accelerating, braking, and changing gears.

2. Open-Closed Principle (OCP): The `Car` class should be open for extension but closed for modification. This means that we should be able to add new features or behaviors to the `Car` class without modifying its existing code. For example, we can create a new class called `ElectricCar` that inherits from the `Car` class and adds additional behavior specific to electric cars, such as charging.

3. Liskov Substitution Principle (LSP): Objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program. In our example, if we have a method that accepts a `Car` object as a parameter, we should be able to pass an instance of `ElectricCar` without any issues.

4. Interface Segregation Principle (ISP): Clients should not be forced to depend on interfaces they do not use. Instead of having a single interface with many methods, we should have multiple smaller interfaces that are specific to the needs of the clients. For example, instead of having a single `ICar` interface with methods for accelerating, braking, and changing gears, we can have separate interfaces like `IAcceleratable`, `IBrakable`, and `IGearChangeable`.

5. Dependency Inversion Principle (DIP): High-level modules should not depend on low-level modules. Both should depend on abstractions. In our example, instead of directly instantiating dependencies within the `Car` class, we can use dependency injection to provide the required dependencies. This allows for easier testing and decouples the `Car` class from its dependencies.

By following these SOLID principles, we can create more maintainable, flexible, and scalable software systems.