Summary - Abstract classes

C# course · Intermediate · Lesson 58/116

Combine abstract methods and override to write a shape hierarchy.

Summary - Abstract classes

In this group you learned abstract classes: an abstract class cannot be instantiated, an abstract method is bodyless and must be overridden, virtual is bodied and optional, a hierarchy provides a common template and sealed blocks inheritance or further overrides.

Abstract classes share common behavior and enforce a contract on subclasses. The next group dives into polymorphism: polymorphic behavior, method hiding and pattern matching.

Exercise Writer class (summary)

Add a public abstract void Write(string message); method to Writer. ConsoleWriter and Main are already in place; you only add the abstract method.

Show example solution

Add `public abstract void Write(string message);` to Writer. In Main write `Writer w = new ConsoleWriter(); w.Write("Hello");`.
Example solution:
using System;

abstract class Writer
{
    public abstract void Write(string message);
}

class ConsoleWriter : Writer
{
    public override void Write(string message)
    {
        Console.WriteLine(message);
    }
}

class Program
{
    static void Main()
    {
        Writer w = new ConsoleWriter();
        w.Write("Hello");
    }
}

Exercise Cat class (summary)

Define the Cat class: it inherits Animal and overrides Sound; the body returns "Meow". In Main, create Animal a = new Cat(); and call a.Greet();.

Show example solution

Animal { public override string Sound() { return "Meow"; } }`. In Main write `Animal a = new Cat(); a.Greet();`.
Example solution:
using System;

abstract class Animal
{
    public abstract string Sound();

    public void Greet()
    {
        Console.WriteLine("Sound: " + Sound());
    }
}

class Cat : Animal
{
    public override string Sound()
    {
        return "Meow";
    }
}

class Program
{
    static void Main()
    {
        Animal a = new Cat();
        a.Greet();
    }
}

Next step

Abstract classes provide common template and contract; an abstract method forces behavior on subclasses, virtual gives flexibility, sealed sets a boundary. The next group covers polymorphism: calling different subclass behaviors through an abstract class instance, method hiding with new and type checks with is/as.

Remember: an abstract class says "is-a"; an interface says "can-do". Together they enable powerful designs.

Test yourself

Which statement about abstract classes is correct?
- An abstract class can contain both bodied and bodyless methods.
- An abstract class can be instantiated with new.
- Abstract methods can have a body.
- A sealed abstract class can be defined.
Show answer

An abstract class can contain both bodied and bodyless methods. — Correct. An abstract class can contain both bodied and bodyless methods.

Fill in the blank: abstract method signature

abstract class Writer
{
    public ____ void Write(string message);
}

Show answer

abstract class Writer
{
    public abstract void Write(string message);
}

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Summary - Abstract classes