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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 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? 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 interface, and what can (and can't) it contain in its most basic form?

Beginner

Answer

An interface defines a CONTRACT of members (methods, properties, events, indexers) that implementing classes/structs must provide — traditionally, an interface couldn't contain ANY implementation, only signatures, though modern C# (8+) allows optional default implementations too.
public interface IShape {
  double CalculateArea(); // no implementation, just a contract
  double Perimeter { get; } // property signature only
}
public class Circle : IShape {
  public double Radius { get; set; }
  public double CalculateArea() => Math.PI * Radius * Radius;
  public double Perimeter => 2 * Math.PI * Radius;
}
Real-world example Defining a common IShape contract that Circle, Square, and Triangle all implement independently.

Common follow-ups: Can a class implement MULTIPLE interfaces at once, unlike single inheritance from a base class?

OOP
All 17 Interfaces & Abstract Classes questions