What is an abstract class?

An abstract class is a class that cannot be instantiated directly. It serves as a base class for other classes and can contain abstract members (methods, properties, indexers, events) that must be implemented in derived classes.

Main features:

• Declared with the keyword abstract.
• Can contain both abstract and non-abstract (regular) members.
• Abstract members do not have an implementation in the base class.
• A derived class must either implement all abstract members of the base class or be declared as abstract itself.

It is used to define a common contract for a group of related classes and to implement the Template Method pattern.

// Declaration of an abstract class
public abstract class Shape
{
    // Abstract method - has no implementation
    public abstract double Area();

    // Non-abstract method
    public void Display()
    {
        Console.WriteLine("This is a shape.");
    }
}

// Derived class inheriting from the abstract class
public class Circle : Shape
{
    public double Radius { get; set; }

    // Implementation of the abstract method Area
    public override double Area()
    {
        return Math.PI * Radius * Radius;
    }
}