66. Overriding#

Method overriding allows a subclass to provide its unique implementation for a method already defined in its superclass. This is a form of subtype polymorphism that allows the subclass to inherit a behavior and then tailor it to its specific needs.

Warning

Don’t confuse method overriding with method overloading.

To demonstrate, let’s consider an Animal class with a Speak method:

public class Animal
{
    public virtual void Speak()
        => Console.WriteLine("The animal makes a sound.");
}

Notice how the Speak method is marked as virtual. This means that it can be overridden in any subclass. Beyond that the method works just like any other method.

Animal animal = new Animal();
animal.Speak();

The animal makes a sound.

Now, let’s define a Dog class that inherits from Animal and overrides the Speak method:

public class Dog : Animal
{
    public override void Speak()
        => Console.WriteLine("The dog barks.");
}

By marking the Speak method with the override keyword, we’re signaling that this method is intended to replace the implementation of a method with the same signature in the superclass. Now, if we create a Dog object and call its Speak method, we’ll see that it runs the implementation specified in Dog rather than Animal.

Dog dog = new Dog();
dog.Speak();

The dog barks.

Even when the Dog object is stored in a variable of type Animal, the overridden Speak method of the Dog class is invoked:

Animal animal = new Dog();
animal.Speak();

The dog barks.

This is because the run-time type of the object determines which implementation to call, not the compile-time type. Since the run-time type of the object is Dog the run-time system dispatches to the Dog’s Speak method.

Remember

Which implementation is executed is determined by the run-time type and not the compile-time type. Return to the chapters on dynamic dispatch and run-time types vs compile-time types if this doesn’t seem familiar.

However, not all methods are created equal, and not all of them can be overridden. Only methods marked with the virtual keyword in the superclass can be overridden in the subclass. If a method in the superclass is not declared as virtual, the subclass can’t override it.

Note

In C# methods are not overridable by default. This behavior varies from language to language. In Java we mark methods that cannot be overridden instead of those that can.

Furthermore, a method marked as override in a subclass is also virtual by default and can be further overridden in any subclasses of that subclass unless it is explicitly marked as sealed.

Key point

Method overriding allows a subclass to provide a specific implementation of a method that’s already provided by its superclass. The overridden method is selected based on the run-time type of the object, not its compile-time type.

In the next chapter, we’ll dive into ‘Hiding’, an approach that enables a subclass to obscure a superclass method without altering its original behavior when accessed through superclass references.