Interfaces and Abstract Classes in C#

# CHAPTER 18

Interfaces and Abstract Classes #

1. Introduction #

2. Learning Objectives #

• Define and implement an Interface.

• Differentiate between Interfaces and Abstract Classes.

• Implement Multiple Interfaces in a single class.

• Understand the concept of Dependency Abstraction.

3. What is an Interface? #

By naming convention, Interfaces in C# always start with a capital I (e.g., IMovable, IDrawable).

using System;

// Defining an Interface
interface IPlayable
{
    // Interface methods have NO body and no access modifiers (they are public by default)
    void Play();
    void Pause();
}

// Implementing the Interface
class VideoPlayer : IPlayable
{
    // Must provide public implementations for all interface methods!
    public void Play()
    {
        Console.WriteLine("Playing Video...");
    }

    public void Pause()
    {
        Console.WriteLine("Video Paused.");
    }
}

4. Multiple Interface Implementation #

interface ICamera
{
    void TakePhoto();
}

interface IGPS
{
    void GetLocation();
}

// Inherits ONE Base class (Device), implements TWO Interfaces (ICamera, IGPS)
class SmartPhone : Device, ICamera, IGPS
{
    public void TakePhoto() { Console.WriteLine("Click!"); }
    public void GetLocation() { Console.WriteLine("Lat: 40, Long: -74"); }
}

5. Abstract Classes vs. Interfaces (The Golden Interview Question) #

6. Dependency Abstraction #

interface ILogger { void Log(string msg); }

class ConsoleLogger : ILogger { public void Log(string msg) { Console.WriteLine(msg); } }
class DatabaseLogger : ILogger { public void Log(string msg) { /* SQL Logic */ } }

class Application
{
    // Depends on the Interface, not the concrete class!
    public void Run(ILogger logger)
    {
        logger.Log("App Started!");
    }
}

7. Common Mistakes #

• Adding access modifiers inside an Interface: In classic C#, writing public void Play(); inside an interface will cause an error. Just write void Play();.

• Forgetting to implement all methods: If a class claims to implement IPlayable, it MUST contain code for every single method defined in IPlayable. The compiler will not let you compile otherwise.

8. Best Practices #

• Always prefix your Interface names with the letter I (e.g., IRepository, IUserService).

• Interface names are often adjectives (IMovable, IComparable) because they define capabilities rather than core identities.

9. Exercises #

1. 1. Create an interface IShape with a double GetArea() method. Implement it in a Rectangle class.

1. 2. Create an interface IFlyable and an interface ISwimmable. Create a Duck class that implements both.

10. MCQs with Answers #

Q5. Are interface methods implicitly public? a) Yes b) No, they are private by default Answer: a) Yes

Q6. If a class implements an interface, must it provide code for ALL methods in that interface? a) Yes b) No, only the ones it needs Answer: a) Yes

Q7. Which represents an "Is-A" relationship? a) Interface (e.g., IFlyable) b) Base Class (e.g., Animal) Answer: b) Base Class (e.g., Animal)

Q8. Which represents a "Can-Do" capability? a) Interface (e.g., IFlyable) b) Base Class (e.g., Animal) Answer: a) Interface (e.g., IFlyable)

11. Interview Questions #

• Q: What is the difference between an Interface and an Abstract Class? Give 3 differences.

• Q: Explain why C# prohibits multiple class inheritance, but allows multiple interface implementation (The "Diamond Problem").

12. Summary #

13. Next Chapter Recommendation #