Introduction
When you start learning object-oriented programming in C#, two important concepts you will quickly come across are Interfaces and Abstract Classes. At first, many beginners feel confused because both seem to do similar things: they allow you to define a structure and force child classes to follow certain rules.
However, Interfaces and Abstract Classes are not the same, and choosing the correct one is extremely important when designing software architecture.
This article explains:
The goal is to help you understand these concepts in the simplest possible way so you can confidently use them in real projects.
Understanding Interface
What Is an Interface?
An interface in C# is like a contract that defines what methods or properties a class must implement. However, the interface itself does not provide any implementation.
Think of an interface as a promise. If a class implements an interface, it must fulfil all the methods defined inside the interface.
Real-World Example of Interface
Imagine a USB port on a computer.
A USB port defines rules such as:
Shape of the connector
Voltage
Signal type
A USB port does not care:
As long as the device follows the USB contract, it will work.
Similarly, an interface defines the structure, not the implementation.
Understanding Abstract Class
What Is an Abstract Class?
An abstract class is a class that cannot be instantiated directly. It may contain:
Fully implemented methods
Partially implemented methods
Abstract methods (without body)
Abstract classes are used when you want to provide common functionality to multiple related classes while also enforcing certain rules.
Real-World Example of Abstract Class
Consider a general concept: Vehicle.
All vehicles share common features:
All have wheels
All have engines
All need fuel
All have a speed
But each type of vehicle behaves differently:
A car accelerates in one way
A bike accelerates differently
A truck accelerates differently
So you can create an abstract Vehicle class, give it:
Shared properties like Speed
Shared methods like StartEngine
Abstract methods like Accelerate
Each child class will provide its own version of Accelerate.
Interface vs Abstract Class: Programming Examples
Interface Example
public interface IAnimal
{
void Eat();
void Speak();
}
Now, any class implementing this interface must follow the contract:
public class Dog : IAnimal
{
public void Eat()
{
Console.WriteLine("Dog is eating.");
}
public void Speak()
{
Console.WriteLine("Dog barks.");
}
}
public class Cat : IAnimal
{
public void Eat()
{
Console.WriteLine("Cat is eating.");
}
public void Speak()
{
Console.WriteLine("Cat meows.");
}
}
Here, both Dog and Cat obey the IAnimal contract.
Abstract Class Example
public abstract class Vehicle
{
public int Speed { get; set; }
public void StartEngine()
{
Console.WriteLine("Engine started.");
}
public abstract void Accelerate();
}
Child classes:
public class Car : Vehicle
{
public override void Accelerate()
{
Speed += 10;
Console.WriteLine("Car speed: " + Speed);
}
}
public class Bike : Vehicle
{
public override void Accelerate()
{
Speed += 5;
Console.WriteLine("Bike speed: " + Speed);
}
}
This shows how shared behaviour comes from the abstract class and unique behaviour from the child classes.
Real-World Analogy: Interface vs Abstract Class
| Concept | Interface | Abstract Class |
|---|
| Analogy | A rulebook or contract | A partially completed blueprint |
| Defines | Only rules | Shared behaviour + rules |
| Relationship | Can be implemented by any unrelated class | For classes with common ancestry |
| Example | USB port rules | Base vehicle class |
Technical Differences Between Interface and Abstract Class
1. Implementation
Interface
Cannot have implementation (until C# 8 introduced default methods, but still limited).
Abstract Class
Can have both abstract and non-abstract methods.
2. Inheritance
3. Constructors
4. Use Cases
Interface
For defining capability or behaviour (like IPrintable, ICloneable).
Abstract Class
For defining a parent class with shared logic.
5. Fields
When Should You Use an Interface?
Use an interface when:
You want to define a capability
Examples:
IFlyable
IRunnable
ISerializable
You want multiple inheritance of behaviour
A class can implement many interfaces.
You want to make components loosely coupled
Helps in Dependency Injection.
You want different unrelated classes to follow the same rules
Example:
You want to achieve contract-based design
Useful in service layers, repositories, and API designs.
When Should You Use an Abstract Class?
Use an abstract class when:
You want to build a base class
Example:
Animal, Vehicle, Employee.
You want to provide shared functionality
Example: methods like StartEngine, Login, Validate.
You expect all child classes to share common behaviour
Abstract class works best for related classes.
You need fields, constructors, or full method implementations
Interfaces do not provide these features.
You are designing a hierarchical system
For example:
Vehicle → Car → SportsCar
Real-World Business Example
Scenario
You are designing a payment system for an e-commerce application.
Abstract Class Approach
Create a base class called Payment:
public abstract class Payment
{
public decimal Amount { get; set; }
public void ValidateAmount()
{
if (Amount <= 0)
throw new Exception("Invalid amount");
}
public abstract void Pay();
}
Child classes:
CardPayment
WalletPayment
NetBankingPayment
They all share common logic like validation but implement their own Pay() method.
Interface Approach
Define capabilities:
public interface IRefundable
{
void Refund();
}
Now only some payment methods implement Refund, not all.
This is a perfect use case for interface-based capability.
Side-by-Side Comparison Table
| Feature | Interface | Abstract Class |
|---|
| Methods | Only signatures (no body) | Can have full method implementations |
| Fields | Not allowed | Allowed |
| Constructors | Not allowed | Allowed |
| Multiple Inheritance | Allowed | Not allowed |
| Use Case | Define capability | Define base class |
| Supports Access Modifiers | Limited | Full support |
| Code Reuse | No | Yes |
Which Is Better to Use in C#?
There is no universal answer.
Both serve different purposes.
However, here is the general guideline:
Use Interface When
You want a simple contract
You need loose coupling
You expect multiple unrelated classes to share behaviour
You want to support dependency injection
Use Abstract Class When
You have a parent-child relationship
You want to share common logic
You need constructors, fields, or partial implementation
Your classes are closely related
Modern C# Note
Interfaces have become more powerful since C# 8 added default methods.
However, abstract classes still provide:
Constructors
Fields
Protected members
Better code reuse
Thus, abstract class remains the best choice for common base functionality, while interfaces remain best for contracts.
Summary
Interfaces and abstract classes both help structure applications, but their purpose is different.
Interface
A contract that defines what a class must do, without specifying how it must do it.
Abstract Class
A partially implemented base that provides shared logic as well as rules for child classes.
Which One Should You Choose?
If you want capability, choose interface.
If you want reusable code, choose abstract class.
If you want multiple inheritance, choose interface.
If your classes share common behaviour, choose abstract class.
Both concepts are equally important, and understanding when to use each is essential for writing clean, professional C# code.