What is the difference between abstract class and interface in C#?

An abstract class can provide implementation, fields, and constructors, while interfaces are contracts with no implementation prior to C# 8. A class can implement multiple interfaces but only inherit one abstract class.

When should I use an interface instead of an abstract class?

Use an interface when unrelated classes need to follow the same contract and when you want to maximize flexibility and testability.

Difference Between Abstract Class and Interface in C# | FullStackPrep.Dev

💡 Concept: Abstract Class vs Interface

Abstract classes and interfaces are used to define contracts in object-oriented programming. While both allow defining methods that derived classes must implement, they serve different purposes and follow distinct rules in C#.

📘 Quick Intro

An abstract class allows partial implementation and can include fields and constructors. An interface is a pure contract that specifies what a class should do, but not how. Interfaces allow multiple inheritance, whereas classes support only single inheritance.

🧠 Analogy

Think of an abstract class as a partially built building with basic utilities like plumbing installed, while an interface is a checklist of items (doors, windows, rooms) that the builder must implement from scratch. Abstract classes help provide structure; interfaces help enforce rules.

🔧 Technical Explanation

Abstract Class: Can have implementation, fields, and constructors.
Interface: Only declares members (until C# 8, where default implementations are allowed).
Inheritance: A class can inherit one abstract class but implement multiple interfaces.
Access Modifiers: Abstract class members can have access modifiers; interface members are public by default.
Polymorphism: Both support polymorphism, but interfaces offer greater flexibility for loose coupling.

🎯 Use Cases

✅ Use interfaces to define contracts across unrelated classes.
✅ Use abstract classes when you need a base class with default implementations.
✅ Abstract classes are useful for shared functionality; interfaces for plug-and-play behavior.
✅ Use interfaces with dependency injection and unit testing frameworks.

💻 Code Example

// Abstract class example
public abstract class Vehicle {
    public abstract void Start();
    public void Stop() {
        Console.WriteLine(""Stopping..."");
    }
}

// Interface example
public interface IElectric {
    void Charge();
}

// Derived class implementing both
public class Tesla : Vehicle, IElectric {
    public override void Start() {
        Console.WriteLine(""Starting electric vehicle..."");
    }

    public void Charge() {
        Console.WriteLine(""Charging battery..."");
    }
}

❓ Interview Q&A

Q1: Can abstract classes have fields?
A: Yes, they can.

Q2: Can interfaces have fields?
A: No, interfaces cannot have fields.

Q3: Can interfaces define constructors?
A: No, constructors are not allowed in interfaces.

Q4: Can we create an object of an abstract class?
A: No, abstract classes cannot be instantiated.

Q5: Can an abstract class implement an interface?
A: Yes, and it may defer implementation to subclasses.

Q6: What is the default access modifier of interface members?
A: Public.

Q7: Can abstract classes include implemented methods?
A: Yes.

Q8: Can interfaces have default methods in modern C#?
A: Yes, from C# 8 onward.

Q9: Can a class extend an abstract class and implement interfaces?
A: Yes.

Q10: Can we override a non-abstract method from abstract class?
A: Only if it’s marked virtual.

📝 MCQs

Q1. What is a key feature of abstract classes?

They can’t be inherited
Only for static members
They can have method implementations
None

Q2. What’s true about interfaces (C# < 8)?

Can have fields
No implementation allowed
Have constructors
Access modifiers supported

Q3. Which supports multiple inheritance?

Abstract classes
Interfaces
Static classes
Delegates

Q4. Can an abstract class have constructors?

No
Yes
Only private
Only static

Q5. Can interfaces contain properties?

No
Yes
Only static
Only inherited

Q6. Which is better for DI and mocking?

Abstract classes
Interfaces
Static classes
Records

Q7. Can abstract classes define private methods?

No
Yes
Only virtual
Only protected

Q8. What’s used for common behavior and partial implementation?

Interface
Record
Static class
Abstract class

Q9. What is required to override an abstract method?

Use partial
Use sealed
Must be in a derived class
Must be private

Q10. Interfaces members default to which access?

Private
Protected
Internal
Public

💡 Bonus Insight

You can combine both: use abstract class for shared implementation and interfaces to define additional contracts, giving the best of both inheritance and flexibility.

📄 PDF Download

Need a handy summary for your notes? Download this topic as a PDF!

📄 PDF Download Coming Soon

Difference Between Abstract Class and Interface in C#