Abstract Class vs Interface

In C#, Abstract Class and Interface are both used for abstraction, but they serve different purposes.

Quick Difference (Interview Answer)

Abstract Class → Used when classes share common behavior + base implementation
Interface → Used when classes share only contract (no implementation)

1. Abstract Class

An Abstract Class can have:

✅ Abstract methods
✅ Concrete methods
✅ Fields
✅ Constructors
✅ Properties

Example

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

    public abstract void MakeSound();
}

Child class:

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

Usage

Dog dog = new Dog();
dog.Eat();
dog.MakeSound();

2. Interface

An Interface only defines contract (what to implement)

Example

public interface IAnimal
{
    void Eat();
    void MakeSound();
}

Child class:

public class Dog : IAnimal
{
    public void Eat()
    {
        Console.WriteLine("Dog is eating");
    }

    public void MakeSound()
    {
        Console.WriteLine("Dog barks");
    }
}

Key Differences Table

Feature	Abstract Class	Interface
Methods with body	✅ Yes	⚠️ Yes (C# 8+)
Abstract methods	✅ Yes	✅ Yes
Fields	✅ Yes	❌ No
Constructors	✅ Yes	❌ No
Access Modifiers	✅ Yes	❌ No (Mostly public)
Multiple inheritance	❌ No	✅ Yes
When to use	Base class	Contract

Important Interview Question

Multiple Inheritance

Abstract class ❌ Not allowed

class A {}
class B {}

class C : A, B   // ❌ Not allowed
{
}

Interface ✅ Allowed

interface IA {}
interface IB {}

class C : IA, IB   // ✅ Allowed
{
}

When to Use Abstract Class

Use Abstract Class when:

✅ Classes are closely related
✅ Want common functionality
✅ Need shared code

Example:

Animal → Dog, Cat
Employee → Manager, Developer

When to Use Interface

Use Interface when:

✅ Different classes share behavior
✅ Need multiple inheritance
✅ Want loose coupling

Example:

ILogger
IRepository
IEmailService

Real-World Example

interface ILogger
{
    void Log(string message);
}

class FileLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine("File log");
    }
}

class DbLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine("DB log");
    }
}

Interview One-Line Answer

Abstract class provides partial implementation, while Interface provides only contract. Abstract class supports single inheritance, Interface supports multiple inheritance.

Trick Interview Question

Q: Can interface have implementation?

👉 Yes, after C# 8.0 (Default Interface Methods)

public interface ITest
{
    void Test()
    {
        Console.WriteLine("Default implementation");
    }
}