Difference between Abstract Class and Interface in C#

Introduction

When learning C#, one of the most common and important questions developers ask is: What is the difference between an abstract class and an interface?

Both abstract classes and interfaces are used to achieve abstraction, which means hiding implementation details and showing only essential features. They are widely used in object-oriented programming (OOP) to design clean, scalable, and maintainable applications.

In this article, we will understand the difference between an abstract class and an interface in C# using simple language, real-world examples, and practical code.

What is an Abstract Class in C#?

An abstract class is a special type of class that cannot be instantiated (you cannot create its object directly). It is designed to be inherited by other classes.

An abstract class can contain:

Abstract methods (without implementation)
Non-abstract methods (with implementation)
Fields (variables)
Constructors

In simple words, an abstract class provides a base structure along with some default behavior.

Example of Abstract Class

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

    public abstract void Drive();
}

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

Explanation:

Vehicle is an abstract class
It has a normal method Start()
It has an abstract method Drive()
Car class inherits and implements the Drive() method

What is an Interface in C#?

An interface is like a contract. It defines what a class must do, but not how it does it.

In C#, an interface contains only method signatures (by default), properties, events, or indexers, but no implementation (except in newer versions with default methods).

A class that implements an interface must implement all its members.

Example of Interface

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

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

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

Explanation:

IVehicle is an interface
It only defines methods
Bike class implements all methods

Key Differences Between Abstract Class and Interface in C#

Feature	Abstract Class	Interface
Definition	A base class with partial implementation	A contract with no implementation
Methods	Can have both abstract and non-abstract methods	Only method declarations (mostly)
Fields	Can have fields	Cannot have fields
Constructors	Allowed	Not allowed
Multiple Inheritance	Not supported	Supported
Access Modifiers	Can use access modifiers	Members are public by default
Usage	Used for closely related classes	Used for unrelated classes

Real-World Example to Understand Clearly

Let’s understand with a real-world scenario.

Abstract Class Example (Same Family)

Think of a base class Employee:

public abstract class Employee
{
    public string Name { get; set; }

    public void Work()
    {
        Console.WriteLine("Employee is working...");
    }

    public abstract void CalculateSalary();
}

public class FullTimeEmployee : Employee
{
    public override void CalculateSalary()
    {
        Console.WriteLine("Calculating full-time salary...");
    }
}

Here:

All employees share common behavior
Some logic is already defined
Some logic is different and must be implemented

Interface Example (Different Capabilities)

Now think of an ability like IFlyable:

public interface IFlyable
{
    void Fly();
}

public class Bird : IFlyable
{
    public void Fly()
    {
        Console.WriteLine("Bird is flying...");
    }
}

public class Airplane : IFlyable
{
    public void Fly()
    {
        Console.WriteLine("Airplane is flying...");
    }
}

Here:

Different objects (Bird, Airplane)
Same capability (Fly)
No shared base class needed

When to Use Abstract Class in C#

Use an abstract class when:

You want to share common code among related classes
You need constructors or fields
You want partial implementation

Example: Employee, Vehicle, Shape

When to Use Interface in C#

Use an interface when:

You want to define a contract
Classes are not closely related
You need multiple inheritance

Example: ILogger, IDisposable, IFlyable

Advantages of Abstract Class

Code reuse
Better structure for related classes
Can include implementation

Advantages of Interface

Supports multiple inheritance
Promotes loose coupling
Flexible design

Summary

Abstract classes and interfaces are both important concepts in C# for achieving abstraction and building scalable applications. An abstract class is best when you want to share common functionality among related classes, while an interface is ideal when you want to define a contract that multiple unrelated classes can implement. Understanding when to use each helps you write cleaner, more maintainable, and flexible code in real-world applications.