abstract methods

C# course · Intermediate · Lesson 54/116

Learn abstract methods and overriding in derived classes.

Abstract methods

An abstract method is a method signature with no body that must be overridden in a subclass. Declaration: public abstract int Area();.

Abstract methods can only live inside an abstract class. If the subclass does not write a body as public override int Area() { ... }, you get a compile error. This guarantees that subclasses implement a specific behavior. Abstract methods cannot be private because the subclass must see them.

Override requirement

Every concrete class that inherits an abstract class must override all its abstract methods. Otherwise the compiler errors out.

Example: if you declare abstract class Shape { public abstract int Area(); }, then inside class Square : Shape you must write public override int Area() { return Side * Side; }. If you don't, you must mark Square itself as abstract. This rule guarantees the contract is fully implemented.

Exercise Square area

Add a public abstract int Area(); method to Shape. In Square write the public override int Area() body: return Side * Side;. In Main, create Shape s = new Square { Side = 4 }; and print s.Area().

Show example solution

Add `public abstract int Area();` to Shape. Add `public override int Area() { return Side * Side; }` to Square. In Main write `Shape s = new Square { Side = 4 }; Console.WriteLine(s.Area());`.
Example solution:
using System;

abstract class Shape
{
    public abstract int Area();
}

class Square : Shape
{
    public int Side { get; set; }

    public override int Area()
    {
        return Side * Side;
    }
}

class Program
{
    static void Main()
    {
        Shape s = new Square { Side = 4 };
        Console.WriteLine(s.Area());
    }
}

Test yourself

Match: abstract method parts

abstract void MakeSound

bodyless signature
override

body in subclass
missing override

compile error
public

default access

Fill in the blank: abstract method

abstract class Shape
{
    public ____ int Area();
}

Show answer

abstract class Shape
{
    public abstract int Area();
}

Which statement about abstract methods is correct?
- An abstract method can have a body.
- An abstract method can be private.
- An abstract method is not redeclared in the implementing class.
- An abstract method is bodyless and is overridden in the implementing class.
Show answer

An abstract method is bodyless and is overridden in the implementing class. — Correct. An abstract method is bodyless and override is required.

abstract methods