LESSON 644 OF 674 · OBJECT-ORIENTED PROGRAMMING

Interfaces in C#

Lesson

TL ; DR:

An interface is a pure contract — a list of methods that a class promises to implement, with no shared data and no shared code. Unlike inheritance ("is a"), interfaces express "can do".

interface ISpeaker {
    void Speak();
}
class Dog : ISpeaker {
    public void Speak() {
        Console.WriteLine("Woof");
    }
}

Full lesson:

An interface defines what a class can do, not how it does it or what it is. A Truck, a Robot, and a Drone aren't the same kind of thing — but they can all Move(). You'd put Move() in an IMovable interface and have each one implement it.

Interface vs abstract class

	Abstract class	Interface
Has fields	Yes	Usually no
Has method bodies	Yes (some)	No (or default impls only)
Multiple "inheritance"	Usually no	Yes — a class can implement many interfaces
Models	"is a"	"can do"

A class inherits from at most one parent class but can implement many interfaces. That's why interfaces are the right tool for cross-cutting capabilities like IComparable, IDisposable, IEnumerable, etc.

In C#

Interfaces use the interface keyword, and by convention their names start with I. C# uses : both for inheriting from a class and for implementing interfaces; a class lists its base class first, then any interfaces.

Your task

The ISpeaker interface is already declared. Create a Dog class that implements it, with a Speak() method that prints "Woof".

Expected output:

Woof

STEP 1 OF 2

Guided

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