C# Interface vs Abstract Class – Complete Guide

Introduction

C# supports both Interfaces and Abstract Classes as ways to define contracts and base functionality. Choosing the right one is crucial for maintainable and reusable code.

1. Interface in C#

An interface defines a contract with method signatures but no implementation. Classes that implement the interface must provide implementations for all its members.

public interface IShape {
    void Draw();
    double CalculateArea();
}

2. Abstract Class in C#

An abstract class can include both abstract methods (without implementation) and concrete methods (with implementation). Derived classes must implement abstract methods.

public abstract class Shape {
    public abstract void Draw();
    public void DisplayShape() {
        Console.WriteLine("This is a shape");
    }
}

3. Key Differences

• Interfaces cannot contain fields, abstract classes can.
• Interfaces allow multiple implementation, classes can inherit only one abstract class.
• Interfaces define only behavior, abstract classes can provide default behavior.
• Interfaces support default methods starting C# 8.0.

4. When to Use

Use Interface when you want multiple unrelated classes to share behavior. Use Abstract Class when sharing code among closely related classes.

Frequently Asked Questions – Interface vs Abstract Class in C#