Wednesday, July 08, 2020 21:40

Table of contents >> Object Oriented Programming > Virtual Methods

Virtual Methods

Virtual methods are methods that can be overridden in inheriting (derived) classes. By default, in .NET, methods are not virtual. In order to declare a method as virtual, we need to declare it using the keyword virtual, like so:

In this case, any class that inherits from the class where MyMethod() is declared, can also have a method named MyMethod() with the same signature, that will override the method from the parent class. The ability to override virtual methods is a fundamental requirement for polymorphism, of which I will talk in a future lesson.

The method that overrides a virtual method from the parent class must use the keyword override,

This is how we declare the virtual method:

This base class contains the virtual method Hunt(), who’s signature accepts a parameter of type object. This means that any inheriting class that wants to override this method, also needs to declare a method with same name and same signature, like this:

Simple explanation: we have two classes, Cat and Lion, both inheriting from the Felidae base class. They both override the method Hunt(), providing their own custom implementation and printing their personalized messages to the console. Consider the following example:

which produces the following output:

Custom overridden implementation of virtual methodIn the first two instances, we expected to see the Cat and Lion implementation being printed on the console, because we overrode the virtual method of the parent class in the child classes. However, you can notice that even if the third instance was declared of type Felidae, we instantiated it of type Lion, and hence, the overridden method was called, and not the base, virtual one; the console did not print “Felidae hunts gazelles”, it printed “Lion hunts gazelles”. From this, we can conclude that virtual methods, as well as abstract methods, of which I will talk in a future lesson, can be rewritten, overridden. In fact, abstract methods are just virtual methods, but without a concrete implementation. We will also later learn that all methods defined in interfaces are abstract, and therefor virtual, even if this aspect is not explicitly defined.

Now, the logical question comes to mind: what is the use of a method in the base class, if it gets overridden in the child class? Why do we need to declare it in the first place? Well, there are two reasons for this. First of all, we might want to call the virtual method from the child class, without overriding it:

This will produce this output:

Calling a base virtual methodBecause we did not override the Hunt() method in the Cat class, when we called it from the instance of type Cat, we actually called the virtual method of the base class.

A second reason would be the fact that oven when overriding virtual methods in child classes, we can still explicitly call the base virtual implementation of the method:

The console would look like this:

Overriding method calling base virtual methodBecause in the overridden implementation of the Hunt() method we also called base.Hunt(prey), we told the compiler to also explicitly call the virtual implementation of the method, from the base class, while also executing the instructions in the overridden implementation. This means that if we chose to, we can supplementally add the functionality of the base implementation to the functionality of the overridden implementation, effectively executing both.

This is the primary difference between an override method and a normal method. The first one allows us to change/complement/alter some of the inherited functionality. A simple example is the virtual method ToString(), that any .NET object has. Since it is declared as virtual, we can override it in any implementation, providing custom functionality. I can demonstrate this with the following simple example:

The output would be this one:

Default implementation of ToString() virtual method

which is not a very descriptive output. Instead, I can override this default virtual implementation with a custom one, providing a much more humanly readable output:

and the console would print this:

Overridden implementation of virtual method ToString()You have to admit, this is a much nicer way of outputting an object.

One final aspect of virtual methods: if we do not want our overridden methods to be further altered by other classes that would continue to inherit our class, or if we want to stop our child class from being further inherited by other classes, we can use the keyword sealed.  This keyword basically says “this class/property/method CANNOT be inherited”. In this light, it makes no sense to mark a virtual method as sealed. The override keyword tells the compiler that the method can be overridden by child objects, but the sealed keyword says that the method cannot be inherited by a child class in the first place. In fact, when used on methods or properties, the sealed keyword is only allowed on overriding methods or properties.  In that case, the keyword sealed means that the overridden method or property cannot be further inherited/modified by classes inheriting the class where the overriding method is declared:

In the next lesson, I will discuss the abstract concept of programming.

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2 Responses to “Virtual Methods”

  1. Marius M says:

    Nice to see new articles here !

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