All 17 Interfaces & Abstract Classes questions
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Can a class implement two interfaces with the same method? How is it resolved? 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? What is the Interface Segregation Principle and how does it apply in C#? 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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C# › Interfaces & Abstract Classes

When would you choose an abstract class over an interface for extensibility?

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Answer

When you need shared state or non-trivial common implementation, versioning with protected members, or to control the construction of derived types — things interfaces cannot express as cleanly.
public abstract class HttpHandlerBase { protected readonly HttpClient Http; ... }
Real-world example A framework base class that provides plumbing and calls abstract hooks (template method pattern).
All 17 Interfaces & Abstract Classes questions