Difference Between Abstract Class and Interface in C# With Examples

Introduction

In C#, both Abstract Classes and Interfaces are used to achieve abstraction and define a contract for derived classes. They are core concepts in object-oriented programming and are widely used in real-world application development. However, they serve different purposes and understanding the difference between abstract class and interface in C# is important for writing clean, maintainable, and scalable code.

This article explains abstract class vs interface in C# with simple examples, real-world use cases, and a clear comparison table.

What is an Abstract Class in C#?

An abstract class in C# is a class that cannot be instantiated directly. It is designed to be inherited by other classes and can contain both abstract methods (without implementation) and concrete methods (with implementation).

Key Features of Abstract Class

Can contain both abstract and non-abstract methods
Can have constructors
Can have fields (variables)
Supports access modifiers (private, protected, public)
A class can inherit only one abstract class (single inheritance)

Example of Abstract Class

abstract class Vehicle
{
    public string Brand;

    public void Start()
    {
        Console.WriteLine("Vehicle is starting...");
    }

    public abstract void Drive();
}

class Car : Vehicle
{
    public override void Drive()
    {
        Console.WriteLine("Car is driving...");
    }
}

class Program
{
    static void Main()
    {
        Car car = new Car();
        car.Start();
        car.Drive();
    }
}

In this example, the abstract class Vehicle provides a common implementation (Start method) and forces derived classes to implement the Drive method.

What is an Interface in C#?

An interface in C# is a contract that defines what a class must do, but not how it does it. It only contains method signatures, properties, events, or indexers without implementation (except default implementations in newer versions of C#).

Key Features of Interface

Contains only method signatures (by default)
Cannot have constructors
Does not contain fields
Supports multiple inheritance (a class can implement multiple interfaces)
All members are public by default

Example of Interface

interface IVehicle
{
    void Start();
    void Drive();
}

class Car : IVehicle
{
    public void Start()
    {
        Console.WriteLine("Car is starting...");
    }

    public void Drive()
    {
        Console.WriteLine("Car is driving...");
    }
}

class Program
{
    static void Main()
    {
        IVehicle car = new Car();
        car.Start();
        car.Drive();
    }
}

Here, the interface IVehicle defines a contract, and the Car class provides the actual implementation.

Difference Between Abstract Class and Interface in C#

Feature	Abstract Class	Interface
Implementation	Can have both abstract and concrete methods	Mostly only method declarations
Constructors	Allowed	Not allowed
Fields	Can have fields	Cannot have fields
Access Modifiers	Can use different access modifiers	Members are public by default
Inheritance	Supports single inheritance	Supports multiple inheritance
Purpose	Share code among related classes	Define a contract for unrelated classes

When to Use Abstract Class

Use an abstract class when:

You want to share common code among multiple related classes
You need base functionality along with enforced methods
You want to define default behavior

Real-World Example

Consider a banking system where all account types share some functionality like calculating interest or logging transactions. An abstract class can provide base implementation while allowing customization.

When to Use Interface

Use an interface when:

You want to define a contract that multiple classes must follow
You need multiple inheritance
Classes are not closely related but share behavior

Real-World Example

In a payment system, different payment methods like CreditCard, UPI, and NetBanking can implement a common interface like IPayment, ensuring all follow the same structure.

Key Takeaways

Abstract classes are used for code reuse and base functionality
Interfaces are used for defining contracts and achieving flexibility
Abstract classes support single inheritance, while interfaces support multiple inheritance
Interfaces are best for loosely coupled systems

Summary

Understanding the difference between abstract class and interface in C# helps developers design better object-oriented systems. Abstract classes are ideal when you want to share common logic and enforce structure among related classes, while interfaces are best suited for defining contracts across unrelated components. Choosing the right approach depends on your application design, scalability needs, and flexibility requirements.