abstract class
- C# course
- Intermediate
- Lesson 53/116
- 5 min
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.
Practice 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.
Starter code
using System;
// Define the Animal abstract class: Name field and MakeSound abstract method
class Dog : Animal
{
public override void MakeSound()
{
Console.WriteLine("Woof woof");
}
}
class Program
{
static void Main()
{
// Create a dog, set its name and print its sound
}
}Sample 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();
}
}Expected output
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Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Fill in the blank: abstract class
abstract ____
{
public string Name;
public abstract void MakeSound();
}abstract class Animal
{
public string Name;
public abstract void MakeSound();
}Explanation: `class Animal` declares the abstract class.
2 Which statement about abstract classes is correct?
- An abstract class can be instantiated with new.
- An abstract class is designed to be inherited. Correct answer
- An abstract class can only contain methods.
- An abstract class can be sealed.
Explanation: An abstract class is designed to be inherited; it cannot be instantiated directly.
3 Match: abstract class concepts
| Term | Meaning |
|---|---|
abstract class |
designed for inheritance |
new Animal() |
compile error |
abstract method |
bodyless signature |
override |
body in derived class |