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Lesson 2615 min read

Abstract Classes

Learn how abstract classes and abstract methods force derived classes to provide their own implementation.

What is an Abstract Class?

An abstract class cannot be instantiated directly with `new` — it exists only to be inherited from. It can mix fully implemented methods with ones that derived classes must implement themselves.

Abstract Methods

abstract class Shape
{
    public abstract double GetArea(); // no body — must be implemented by derived classes


    public void Describe() // regular method, shared by all shapes
    {
        Console.WriteLine($"This shape has an area of {GetArea():F2}");
    }
}


class Circle : Shape
{
    public double Radius;
    public Circle(double radius) { Radius = radius; }


    public override double GetArea() => Math.PI * Radius * Radius;
}


var circle = new Circle(5);
circle.Describe();
Output

Click Run to see what this code prints.

Why Use Abstract Classes?

They let you define a shared contract and some common behavior at once, while forcing every derived class to supply its own logic for the parts that genuinely differ (like how to calculate area for a Circle vs a Square).

Abstract Class vs Interface

Abstract classes can hold shared implementation and fields; interfaces (next lesson) traditionally can't hold state and are meant purely as a contract. A class can inherit only one abstract class, but implement multiple interfaces.

FAQs

No — `new Shape()` won't compile. Only concrete (non-abstract) derived classes like Circle can be instantiated.

Summary

  • Abstract classes can't be instantiated directly.
  • Abstract methods have no body and must be overridden in derived classes.
  • They combine shared implementation with an enforced contract.
Next Lesson →

Interfaces