What is the difference between an Interface and an abstract class?

This question is frequently asked in the programming field; however, many people answer it in a non-clear way, indicating a weak understanding of Object-Oriented design principles. For instance, some might say

• interfaces don't have implementation, but abstract classes can.
• interfaces don't have constructors, but classes can.
• interfaces can't have data fields, but abstract classes can.

Simply stating these differences may not truly reflect a deep understanding of the distinction between an interface and an abstract class. so even after answering in that way you still don't know when to choose an interface and when to choose an abstract class?

To achieve a deeper understanding, it's crucial to clearly know what an abstract class and an interface are.

An abstract class serves as a mechanism for sharing code among related classes, usually acting as the parent of many classes. The most significant aspect is the implemented methods themselves they representing the default common behavior for all subclasses.

The name of an abstract class should always be a noun, reflecting an existing object (e.g., DBContext, BaseController, Comparer).

Conversely, an interface is nothing but a contract; it defines functionalities that can be implemented by existing entities because it doesn't have any meaning by itself. It is a logical concept, allowing any existing entity like a class or even a struct to implement any number of these contracts. The name of interfaces usually takes the form of an adjective, often ending with "able" (e.g., Runnable, Disposable, Comparable). Understanding when to use an interface and when to use an abstract class involves recognizing the nature of the shared behavior and whether it makes more sense as a common base class or a set of contracts to be implemented.

In summary, if you want to share default behavior among multiple classes, an abstract class is your best choice. If you want to group a set of operations to be executable by any other existing entity like a class or struct, an interface is your best choice.

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