Abstract Classes vs Interfaces in C#: Key Differences Every Developer Should Know for Better OOP Design

Table of Contents

• Introduction
• What is an Abstract Class?
• What is an Interface?
• Key Differences Between Abstract Classes and Interfaces
• Real-World Examples
• When Should You Use What?
• Conclusion

Introduction

Choosing between an abstract class and an interface is one of the most common decisions developers face when designing object-oriented applications in C#. Both play crucial roles in achieving abstraction, reusability, and loose coupling — but they are not interchangeable. Understanding the exact differences can significantly improve your software architecture and prepare you for technical interviews. This guide explores both concepts in depth, explains their features, and provides clean code examples to make your understanding solid and practical.

What is an Abstract Class?

An abstract class is a blueprint for other classes. It can contain both abstract methods (methods without implementation) and non-abstract methods (methods with full implementation). You cannot create an object of an abstract class directly — it must be inherited by another class.

public abstract class Animal
{
    public abstract void MakeSound();

    public void Sleep()
    {
        Console.WriteLine("Sleeping...");
    }
}

public class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Bark");
    }
}

What is an Interface?

An interface defines a contract. Any class that implements an interface must provide implementations for all its members. Unlike abstract classes, interfaces cannot contain implementation (until C# 8 default implementations, but still limited compared to abstract classes).

public interface IAnimal
{
    void MakeSound();
}

public class Cat : IAnimal
{
    public void MakeSound()
    {
        Console.WriteLine("Meow");
    }
}

Key Differences Between Abstract Classes and Interfaces

• Multiple Inheritance: A class can implement multiple interfaces but can inherit only one abstract class.
• Method Implementation: Abstract classes can have both abstract and concrete methods; interfaces contain only abstract methods (with limited default implementations).
• Constructors: Abstract classes can have constructors; interfaces cannot.
• Fields: Abstract classes support fields; interfaces do not (only properties).
• Use Case: Abstract classes represent shared behavior; interfaces represent capabilities.

Real-World Example

Imagine you're building a transportation system. All vehicles share some behaviors, such as starting an engine, but each vehicle may also support different capabilities like flying or floating.

Abstract Class Example

public abstract class Vehicle
{
    public abstract void Start();

    public void Stop()
    {
        Console.WriteLine("Vehicle stopped");
    }
}

Interface Example

public interface IFlyable
{
    void Fly();
}

public interface IFloatable
{
    void Float();
}

When Should You Use What?

Use an Abstract Class when:

• Classes share common functionality.
• You want to provide default implementations.
• You need constructors or fields.

Use an Interface when:

• You want to represent capabilities.
• You expect multiple implementations across different hierarchies.
• You require multiple inheritance of behavior.

Conclusion

Abstract classes and interfaces are powerful tools in C# for achieving clean architecture and reusable code. Abstract classes provide shared behavior, while interfaces define abilities. Understanding these differences helps you design scalable, maintainable applications and answer interview questions confidently.

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