abstract class

Learn to define and derive an abstract class.

Abstract class

An abstract class cannot be instantiated directly and is designed to be inherited. Declaration: abstract class Animal { ... }.

An abstract class can hold both bodied methods and abstract methods (bodyless signatures). Calling new Animal() is a compile error; but you can use it through an instance of a derived class: Animal a = new Dog();. This is useful to share common behavior.

Cannot instantiate but can be inherited

An abstract class cannot be created with new: new Animal() is a compile error. Instead you define a subclass and instantiate it: class Dog : Animal { ... }.

The subclass must implement all the abstract class's abstract methods. Otherwise you get a compile error. This way the abstract class acts as a template for subclasses. Common fields and helper methods live in the abstract class; details are left to subclasses.

Exercise Animal class

Define the Animal abstract class with a public string Name; field and a public abstract void MakeSound(); method. Dog and Main are already in place; you only add the abstract class.

Show example solution

Define Animal as `abstract class Animal { public string Name; public abstract void MakeSound(); }`. In Main write `Animal a = new Dog(); a.Name = "Rex"; Console.WriteLine(a.Name); a.MakeSound();`.
Example solution:
using System;

abstract class Animal
{
    public string Name;

    public abstract void MakeSound();
}

class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Woof woof");
    }
}

class Program
{
    static void Main()
    {
        Animal a = new Dog();
        a.Name = "Rex";
        Console.WriteLine(a.Name);
        a.MakeSound();
    }
}

Test yourself

Fill in the blank: abstract class

abstract ____
{
    public string Name;
    public abstract void MakeSound();
}

Show answer

abstract class Animal
{
    public string Name;
    public abstract void MakeSound();
}

Which statement about abstract classes is correct?
- An abstract class can be instantiated with new.
- An abstract class can only contain methods.
- An abstract class is designed to be inherited.
- An abstract class can be sealed.
Show answer

An abstract class is designed to be inherited. — Correct. An abstract class is designed to be inherited; it cannot be instantiated directly.
Match: abstract class concepts

abstract class

designed for inheritance
new Animal()

compile error
abstract method

bodyless signature
override

body in derived class

Next lesson: abstract methods

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