All 17 Interfaces & Abstract Classes questions
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How do you decide between using an interface versus an abstract class for a given design? How do default interface methods (C# 8+) let an interface provide implementation, and what backward-compatibility problem do they solve? How does explicit interface implementation work, and when would you use it instead of a normal (implicit) implementation? When would you choose an abstract class over an interface for extensibility? How do static abstract interface members (C# 11+) enable generic math and other 'static polymorphism' patterns? How does interface segregation (the 'I' in SOLID) guide splitting a large, monolithic interface into several smaller, more focused ones? How would you design a class hierarchy combining an abstract base class WITH multiple interfaces to model both shared implementation AND multiple independent capabilities? How does the 'sealed' modifier interact with abstract classes and interface method overrides to control further extensibility? How would you implement the 'Liskov Substitution Principle' correctly when designing an abstract class hierarchy, and what's a classic violation example?
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What is the Interface Segregation Principle and how does it apply in C#?

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Answer

Clients should not depend on methods they do not use; prefer several small, focused interfaces over one large one, so implementers and consumers stay decoupled.
// instead of IRepository with 20 methods:
interface IReadRepository<T> { Task<T?> GetAsync(int id); }
Real-world example Splitting a fat service interface so a read-only consumer needn't depend on write methods.
All 17 Interfaces & Abstract Classes questions