interface vs abstract

C# course · Intermediate · Lesson 51/116

Learn the difference between interface and abstract class.

interface vs abstract class

An interface is a contract; it cannot hold fields and contains only method signatures (plus default methods from C# 8). An abstract class can hold both bodied and bodyless methods and fields; it provides partial implementation.

A class can inherit only one abstract class but implement multiple interfaces. An interface represents a "can-do" relationship, an abstract class an "is-a" relationship. Example: Song is a Media (abstract) and is IPlayable (interface).

Using them together

In C# a class can inherit an abstract class and implement multiple interfaces at the same time. Order: abstract class first, then interfaces. Example: class Song : Media, IPlayable, ICloneable.

This pattern is useful to take common behavior from the abstract class and add capabilities with interfaces. Media holds fields and methods common to all media types; IPlayable adds the play capability as a contract.

Exercise Song class

Add a public abstract void Stop(); method to Media. In Song write the public override void Stop() body: Console.WriteLine("Stopped");. In Main, create a Song and call Play and Stop.

Show example solution

Add `public abstract void Stop();` to Media. Add `public override void Stop() { Console.WriteLine("Stopped"); }` to Song. In Main write `Song s = new Song(); s.Play(); s.Stop();`.
Example solution:
using System;

interface IPlayable
{
    void Play();
}

abstract class Media
{
    public abstract void Stop();
}

class Song : Media, IPlayable
{
    public void Play()
    {
        Console.WriteLine("Playing");
    }

    public override void Stop()
    {
        Console.WriteLine("Stopped");
    }
}

class Program
{
    static void Main()
    {
        Song s = new Song();
        s.Play();
        s.Stop();
    }
}

Test yourself

Match: interface vs abstract

interface

contract
abstract class

partial implementation
multiple

interface
single

abstract class
Which statement about interface vs abstract is correct?
- An interface can hold fields, an abstract class cannot.
- An interface is for contracts, an abstract class for shared behavior.
- A class can implement only one interface but inherit multiple abstract classes.
- An interface and an abstract class are the same thing.
Show answer

An interface is for contracts, an abstract class for shared behavior. — Correct. An interface is for contracts, an abstract class for shared behavior.

Fill in the blank: abstract method

abstract class Media
{
    public ____ void Stop();
}

Show answer

abstract class Media
{
    public abstract void Stop();
}

interface vs abstract