All 17 Interfaces & Abstract Classes questions
More in Interfaces & Abstract Classes
Why program to an interface rather than a concrete type? What is an interface, and what can (and can't) it contain in its most basic form? What is an abstract class, and how does it differ from a fully concrete base class? What is the difference between an interface and an abstract class in C#, and when should you choose one over the other? 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? When would you choose an abstract class over an interface for extensibility? What is the Interface Segregation Principle and how does it apply in C#?
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What are default interface methods and when are they useful?

Intermediate

Answer

Since C# 8 an interface can provide a default method body, letting you add members to an interface without breaking existing implementers.
interface ILogger { void Log(string m); void LogError(string m) => Log("ERR: "+m); }
Real-world example Extending a widely-implemented library interface without forcing every implementer to change.
All 17 Interfaces & Abstract Classes questions