Abstract classes and methods in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an abstract class in C#?

An abstract class is a class that cannot be instantiated directly. It is designed to be a base class for other classes and can contain abstract methods that must be implemented by derived classes.

beginner

What is an abstract method?

An abstract method is a method declared in an abstract class without an implementation. Derived classes must provide their own implementation of this method.

beginner

Can you create an object of an abstract class?

No, you cannot create an object of an abstract class directly. You must create an object of a derived class that implements all abstract methods.

intermediate

Why use abstract classes and methods?

They help define a common template for related classes, ensuring that certain methods are implemented in all derived classes, promoting code consistency and reuse.

beginner

Show a simple example of an abstract class with an abstract method in C#.

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

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

Which keyword is used to declare an abstract class in C#?

A abstract

B virtual

C override

D sealed

The keyword 'abstract' is used to declare an abstract class.

Can an abstract method have a body (implementation) in C#?

A Only if the class is sealed

B Yes, always

C No, never

D Only if marked virtual

Abstract methods cannot have an implementation; they only declare the method signature.

What must a derived class do with an abstract method?

A Override and implement it

B Ignore it

C Delete it

D Call base method

Derived classes must override and provide an implementation for all abstract methods.

Can you instantiate an abstract class directly?

A Yes

B Only with new keyword

C Only if it has no abstract methods

D No

Abstract classes cannot be instantiated directly.

Which of these is true about abstract classes?

A They cannot inherit from other classes

B They can contain both abstract and non-abstract methods

C They cannot have constructors

D They can only contain abstract methods

Abstract classes can have both abstract methods and methods with implementations.

Explain what an abstract class and an abstract method are, and why they are useful.

Think about how you create a blueprint for other classes.

Write a simple C# example showing an abstract class with one abstract method and a derived class implementing that method.

Use Animal and Dog as example classes.