Intro to OOP in C#: Inheritance - Page 3
       by kirupa  |  11 February 2007

In the previous page, you learned how to extend a class. In this page, I will explain how to limit access by explaining how to hide inherited methods.

Hiding and Overriding a Method
When extending a class, you will run into situations where the functionality provided by your base class (Character in our case) is not exactly what you want. For example, if you tell your alien to say "X-Files is my favorite show" , you will see something like this: Character says, X-Files is my favorite show.

Let's say you want to modify what your alien says. You could just go and edit the Say method in your Character class, and the end result will be that your alien will now use the modified Say method. There is a problem with this approach though. Because you modified the base Character class, any modification you make to your Character class will also propagate to classes that extend it. For example, your Bandit, Cowboy, and Pirate classes will now inherit your modified Say method also, and you might not want that.

You can avoid this problem by either hiding the unwanted method or by overriding the old method with a new method. Let's set both of those scenarios by simply adding a new Say method inside the Alien class:

class Alien : Character

{

public void Teleport(string currentLocation, string newLocation)

{

Console.WriteLine("The alien teleported from {0} to {1}", currentLocation, newLocation);

}

 

public void Hide()

{

Console.WriteLine("The alien is hiding.");

}

 

public void Say(string thingToSay)

{

for (int i = 0; i < 5; i++)

{

Console.WriteLine("Alien says: {0}", thingToSay);

}

}

}

 

class Program

{

static void Main(string[] args)

{

Character foo = new Character();

foo.Say("Hello World!");

 

Alien zorb = new Alien();

zorb.Say("Take me to your leader!");

}

}

If you run your program (look at the grayed out Program class code for an example) your Alien object will repeat what you tell it to say five times. What you have done is not really override your Say method. You simply hid the Say method from your Character class, and your compiler will throw a warning if you attempt to do something like the following:

class Program

{

static void Main(string[] args)

{

Character zorb = new Alien();

zorb.Say("Take me to your leader!");

}

}

In the above case, you are being ambiguous on which Say method to call. Will your zorb object be calling the Character's Say object or the Alien's Say object? You will actually call the Character's Say method. To be less ambiguous, you can hide methods by using the new keyword instead:

class Alien : Character

{

public void Teleport(string currentLocation, string newLocation)

{

Console.WriteLine("The alien teleported from {0} to {1}", currentLocation, newLocation);

}

 

public void Hide()

{

Console.WriteLine("The alien is hiding.");

}

 

public new void Say(string thingToSay)

{

for (int i = 0; i < 5; i++)

{

Console.WriteLine("Alien says: {0}", thingToSay);

}

}

}

When you add the new keyword to your Say method, you explicitly tell your compiler that the Alien's Say method is intended hide the Say method from the base (Character) class. This is actually the default behavior, so the earlier example without the new keyword worked fine if you chose t ignore the compiler warning.

Overriding a method is a little different though. When you override a method, you tell the compiler to only use the overridden method if possible. In our above example, if we overrode the Say method in our Alien class, our earlier example will use the Say method in the Alien class instead.

Overriding a method takes two steps. You first declare the method you choose the override as virtual. In our example, we declare our Character class's Say method as virtual:

class Character

{

public void Walk()

{

Console.WriteLine("Character walking!");

}

 

public void Talk()

{

Console.WriteLine("Character is talking about something");

}

 

public virtual void Say(string thingToSay)

{

Console.WriteLine("Character says: {0}", thingToSay);

}

}

In the second step, we declare our overriding method in our child class with the override modifier. In our example, the Alien class's Say method will be marked for override:

class Alien : Character

{

public void Teleport(string currentLocation, string newLocation)

{

Console.WriteLine("The alien teleported from {0} to {1}", currentLocation, newLocation);

}

 

public void Hide()

{

Console.WriteLine("The alien is hiding.");

}

 

public override void Say(string thingToSay)

{

for (int i = 0; i < 5; i++)

{

Console.WriteLine("Alien says: {0}", thingToSay);

}

}

}

If you run our earlier program, zorb.Say("Take me to your leader") will now access your Alien class's Say method.

Right now, you are probably wondering what the point of all this is. After all, the first example in this page where I copied a method from the base class without using either new or override worked fine. The issues related to this come up primarily in what is called polymorphism. That is a topic that I will save for a later date, but it is good for you to be aware of how your program's functionality changes during inheritance based on the type of the object calling the inherited method.

Note - Marking Methods as Vritual

When creating your class, unless you believe that a method in it will be overridden, it is best to not leave them declared as virtual. There is a slight performance hit when declaring methods unnecessarily as virtual.

Conclusion
As you can see, inheritance is a very important part of writing software. In software development, you are often told the virtues of having small pieces of reusable code to make writing and maintaining programs easier. An important concept in writing modular code is inheritance.

In this article, you learned how to take a very basic class called Character and extend it. You extended the basic functionality by creating more specialized characters such as our Alien who can both perform everything a character can, but also, perform some unique tricks a generic Character cannot do.

Just a final word before we wrap up. If you have a question and/or want to be part of a friendly, collaborative community of over 220k other developers like yourself, post on the forums for a quick response!

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