Abstract Classes

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();

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).

FAQs

Summary