Interface vs Abstract Class in .NET — Understand the Real Difference Like a Pro!

As a .NET developer, you've likely encountered the question:
“When should I use an Interface, and when should I go for an Abstract Class?”

This isn’t just a textbook concept — it’s a core design decision that can impact your application’s scalability, maintainability, and flexibility. In this post, we’ll dive deep into the difference between Interface and Abstract Class in .NET — with real-world examples, code snippets, and practical use cases.

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🔷 What Is an Interface?

An Interface in .NET is like a contract. It defines what a class must do — but not how it should be done.

Key Characteristics:

• Defines method, property, event, or indexer declarations — without implementation (until C# 8+).

• Supports multiple inheritance (a class can implement multiple interfaces).

• Great for defining roles or capabilities (e.g., IDisposable, IEnumerable).

• Promotes loose coupling and enhances testability.

Example:

public interface IBookable
{
    void BookTicket();
}

Any class implementing IBookable must provide its own logic for BookTicket() — whether it’s a Bus, Train, or Flight.

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🔶 What Is an Abstract Class?

An Abstract Class is a partially implemented class. It allows you to define shared behavior and force child classes to implement specific functionality.

Key Characteristics:

• Can contain both abstract and non-abstract methods.

• Cannot be instantiated directly.

• Does not support multiple inheritance.

• Ideal when you want to define common logic + some customization for subclasses.

Example:

public abstract class Vehicle
{
    public abstract void StartEngine();  // Must be implemented
    public void FuelUp() => Console.WriteLine("Fueling...");
}

Here, FuelUp() provides shared logic, while StartEngine() forces subclasses to implement engine-starting behavior.

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Real-World Example: Transport Booking System

Let’s combine both concepts in a practical scenario:

public interface IBookable
{
    void BookTicket();
}

public abstract class Vehicle
{
    public abstract void StartEngine();
    public void FuelUp() => Console.WriteLine("Fueling...");
}

public class Bus : Vehicle, IBookable
{
    public override void StartEngine() => Console.WriteLine("Bus engine started");
    public void BookTicket() => Console.WriteLine("Bus ticket booked");
}

Explanation:

• IBookable enforces ticket booking behavior.

• Vehicle provides shared logic (FuelUp()) and demands specific implementation for StartEngine().

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Interface vs Abstract Class — At a Glance

When to Use What?

Use an Interface when:

• You want to define a role or capability.

• You need multiple inheritance.

• You're working with unrelated classes that need to implement the same contract.

Use an Abstract Class when:

• You want to provide default behavior.

• Classes share common logic.

• You want to define a base class with some enforced rules.

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🔁 Analogy Time

Choosing between an Interface and an Abstract Class is not just about syntax — it’s about design philosophy:

🔹 Interface = “What should be done?”
🔸 Abstract Class = “How and what should be shared?”

Master this concept and you’ll unlock a powerful tool for building clean, modular, and extensible .NET applications.

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💬 Let’s Chat!

Have you used Interfaces or Abstract Classes creatively in your project?
Drop your story in the comments or link your GitHub!

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Happy coding! ✨

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✍ Written by Rishabh Mishra

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