Lesson 7: Abstract Classes & Methods in C#

Abstract classes allow you to define a template for child classes while preventing direct instantiation. Use them to enforce contracts — child classes must implement certain behavior. This lesson covers abstract classes, abstract methods, and when to use them.

What Are Abstract Classes?

An abstract class is a class marked with the abstract keyword. You cannot instantiate it directly — only classes derived from it can be instantiated:

public abstract class Vehicle
{
    public string Brand { get; set; }

    public abstract void StartEngine();  // No implementation!

    public void Stop()
    {
        Console.WriteLine("Vehicle stopped");
    }
}

// ✗ Cannot create: new Vehicle()
// ✓ Must subclass instead
public class Car : Vehicle
{
    public override void StartEngine()
    {
        Console.WriteLine("Starting car engine");
    }
}

var car = new Car();
car.StartEngine();  // Output: Starting car engine

Abstract Methods

Abstract methods have no implementation — child classes must override them:

public abstract class Shape
{
    public abstract double GetArea();

    public virtual void Display()
    {
        Console.WriteLine($"Area: {GetArea()}");
    }
}

public class Circle : Shape
{
    public double Radius { get; set; }

    public Circle(double radius) { Radius = radius; }

    public override double GetArea()
    {
        return Math.PI * Radius * Radius;
    }
}

var circle = new Circle(5);
circle.Display();  // Output: Area: 78.54...

Abstract Classes vs Interfaces

Use abstract classes for shared behavior. Use interfaces for contracts:

// Abstract class — shared implementation
public abstract class Animal
{
    public void Sleep() { Console.WriteLine("Sleeping"); }
    public abstract void Eat();
}

// Interface — contract
public interface IMovable
{
    void Move();
}

// Usage
public class Dog : Animal, IMovable
{
    public override void Eat() { Console.WriteLine("Eating"); }
    public void Move() { Console.WriteLine("Running"); }
}

🧠 Quick Check — Lesson 7

Can you instantiate an abstract class directly?

Lesson Summary

✅

Abstract classes cannot be instantiated directly — they serve as blueprints for derived classes.

✅

Abstract methods have no implementation; child classes must override them with override.

✅

Abstract classes can contain both abstract and concrete (implemented) methods.

✅

Use abstract classes for shared behavior; use interfaces for contracts and multiple inheritance.

✅

Abstract classes enforce that child classes implement certain behavior, preventing incomplete implementations.

Up Next

Lesson 8: Interfaces in C#

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