virtual and override

Learn to redefine methods with virtual and override.

virtual methods

A virtual method can be rewritten (overridden) in a derived class. The base class offers a default implementation; a subclass may provide its own: public virtual string Sound() { return "..."; }.

The virtual keyword tells the method 'this behavior can change'. A call looks at the runtime type of the object, not the compile-time type of the variable. This is called polymorphism.

override

The derived class rewrites a virtual method of the base class with override: public override string Sound() { return "Woof"; }.

The signature (name, parameters, return type) must match exactly. Redefining without override shadows with new and does not provide polymorphism. For correct behavior use the virtual + override pair. So Animal a = new Dog(); a.Sound(); runs Dog.Sound.

Exercise Override with Shape and Circle

Define a Shape base class with a public virtual double Area() method (body: return 0;). Then declare class Circle : Shape with public double Radius { get; set; } and public override double Area() (body: return 3.14 * Radius * Radius;). Inside Main create Circle c = new Circle { Radius = 2 }; and print Area().

Show example solution

Shape with override Area. In Main write Circle c = new Circle { Radius = 2 }; Console.WriteLine(c.Area());
Example solution:
using System;

class Shape
{
    public virtual double Area()
    {
        return 0;
    }
}

class Circle : Shape
{
    public double Radius { get; set; }

    public override double Area()
    {
        return 3.14 * Radius * Radius;
    }
}

class Program
{
    static void Main()
    {
        Circle c = new Circle { Radius = 2 };
        Console.WriteLine(c.Area());
    }
}

Test yourself

Match each keyword to its role

virtual

Can be overridden
override

Overrides
base

Base class
class

Class definition

Complete the override declaration

public ____ string Sound() { return "Woof"; }

Show answer

public override string Sound() { return "Woof"; }

Animal a = new Dog(); a.Sound(); runs what?
```
// Animal.Sound virtual, Dog.Sound override
```
- Animal.Sound
- Method does not run
- Dog.Sound
- Compile error
Show answer

Dog.Sound — Correct. The runtime type is Dog, so Dog.Sound runs.

Next lesson: sealed

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