C# Abstract Classes vs Interfaces: When to Use

Abstract classes and interfaces in C# are both important concepts used for implementing polymorphism and encapsulation, and they are each suitable for different situations.

1. Abstract class is suitable for classes that need to share some basic implementation logic but want subclass to be able to extend or override this logic according to its own needs. It can contain both abstract and regular methods, and subclasses must implement all abstract methods in order to be instantiated. Abstract classes are often used to define generic behaviors and attributes for specific implementations in subclasses.
2. Interfaces are used to describe the behavior of an object without caring about the object’s internal state or implementation logic. Interfaces define a set of members (methods, properties, events, etc.) and classes that implement the interface must implement these members. Interfaces allow different classes to implement the same behavior, thus achieving polymorphism. Interfaces are typically used to define contracts between objects to ensure they can interact correctly.

In general, abstract classes are suitable for defining shared implementation logic, while interfaces are suitable for defining the behavior contract of objects. Depending on specific needs and design patterns, choosing the appropriate abstract class or interface can enhance the readability and maintainability of the code.