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What is the difference between an interface and an abstract class? 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 the difference between an interface and an abstract class in C#, and when should you choose one over the other? What are default interface methods and when are they useful? 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?
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What is an abstract class, and how does it differ from a fully concrete base class?

Beginner

Answer

An abstract class CANNOT be instantiated directly with 'new' and can contain a mix of concrete (implemented) members AND abstract members (declared but not implemented, requiring subclasses to provide them) — it's meant purely as a base to be extended, unlike a regular concrete class which CAN be instantiated on its own.
public abstract class Animal {
  public string Name { get; set; } = "";
  public abstract void MakeSound(); // must be implemented by subclasses
  public void Sleep() => Console.WriteLine($"{Name} is sleeping."); // shared, concrete implementation
}
// new Animal(); // Error: cannot instantiate an abstract class
Real-world example Defining a shared Animal base class with common behavior (Sleep) plus a required, subclass-specific behavior (MakeSound).

Common follow-ups: Can an abstract class have a constructor, even though it can never be instantiated directly?

Records & Pattern Matching
All 17 Interfaces & Abstract Classes questions