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1 Answers

ICSM Computer
ICSM Computer 12 Jun 2025
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Both abstract classes and interfaces define contracts for other classes to implement, but they differ in capabilities, design purpose, and usage.

Quick Comparison Table

Feature Abstract Class Interface
Inheritance One abstract class per class (single inheritance) Multiple interfaces can be implemented
Members Fields, constructors, methods, properties Only method/property signatures (no fields)
Method implementation Can have implemented & abstract methods C# 8.0+: Can have default methods
Access modifiers Yes (public, protected, etc.) All members are public by default
Constructors Yes No constructors
Use case Common base with shared code Only contract definition (pure abstraction)
Performance/Size Slightly heavier Lightweight

Example: Abstract Class

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

    public void Eat()
    {
        Console.WriteLine("Animal is eating");
    }

    public abstract void MakeSound(); // must be implemented in derived class
}

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

Usage:

Animal dog = new Dog();
dog.Eat();        // Inherited concrete method
dog.MakeSound();  // Overridden method

Example: Interface

public interface IPlayable
{
    void Play();
}

public interface IStoppable
{
    void Stop();
}

public class MediaPlayer : IPlayable, IStoppable
{
    public void Play() => Console.WriteLine("Playing...");
    public void Stop() => Console.WriteLine("Stopped.");
}

Usage:

IPlayable player = new MediaPlayer();
player.Play();

When to Use What?

Scenario Use
Need to provide base functionality Abstract class
Need to support multiple base types Interface
Design a plug-in system or API contract Interface
Need constructors or fields Abstract class
Want to share default code Abstract class (or C# 8+ interfaces)

Tip

  • Use interface when you only need contract/behavior
  • Use abstract class when you need to share logic and structure
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