What purpose does abstract serve in C#?

In C#, the abstract keyword is used to define abstract classes and abstract members, serving the purpose of:

1. Abstract class: An abstract class can be defined using the abstract keyword. It cannot be instantiated and can only be inherited. It can contain both abstract and non-abstract members. Abstract classes are typically used to define common behaviors and attributes of a group of related classes, but cannot be directly instantiated.
2. Abstract members: In abstract classes, you can define an abstract member (method, property, indexer, or event) using the abstract keyword. Abstract members do not have implementation code, only method signatures or property definitions, and need to be overridden with implementation in derived classes. The main purpose of abstract members is to enforce derived classes to provide their own implementation.

By utilizing abstract classes and members, a “contract not concrete implementation” design pattern can be achieved, requiring derived classes to implement specific behaviors and functionalities, ultimately enhancing code maintainability and flexibility.