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One place we can easily see object
oriented programming in action

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is in video games.

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This is because many video games
portray simulations of things

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that might exist in the real world.

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Take a tower defense game for example.

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There are lots of tower
defense games out there.

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They all have a few
things in common though.

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The player places towers on a map,
invaders move down a path and

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the towers shoot at them as they pass by.

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The player wins if the towers
destroy the invaders before they

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can reach the end of the path.

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In order to write this game,
one of the first things we need to do is

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decide what types of objects
the game should contain.

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In other words, we need to determine
what classes we need to code up.

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For this, it's often a good idea to pay
attention to the nouns that are used to

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describe the program.

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Let's take a look at
the description of the game.

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The player places towers on a map,
invaders move down a path, and

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the towers shoot at them as they pass by.

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The player wins if the towers
destroy the invaders before they

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can reach the end of the path.

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Let's see here.

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We have the nouns, player, tower,

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map, invader and path.

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We can model each of these
types of objects in code.

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Let's open work spaces and do it.

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To open work spaces, just click on
the work spaces button on your screen.

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Let's start by creating a class for
our tower.

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The first thing we need
to do is create a file.

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We'll call it tower.cs.

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The code for this game will go in
the treehouse defense namespace.

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If you're unsure about why
I'm using a namespace here,

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I suggest checking out the teachers notes
for a video about namespaces in C#.

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Now, we can declare a class called tower.

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Now, we have a tower class.

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Right now it's just an empty class though.

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Other than the name, there's nothing about
this class that describes what a tower is,

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or how it should behave.

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We still need to add code
to the class to do this.

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It's important to remember that
the tower class is not a tower itself.

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It's just a template for
creating tower objects.

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To create an actual tower object,

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we need to use this class to create
an object of type tower just like we use

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the heart cookie cutter to
create a heart-shaped cookie.

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Let's see how to do that now.

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You can see here, that I've already
created a file called game.cs.

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It contains the definition
of the game class.

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It also contain our main method.

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Remember the main method is a specially
named method that will be run first when

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our program starts.

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This is where we'll create
our first tower object.

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We can do this by first saying
what type of an object it is.

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So, Tower then give the object a name.

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We'll use tower with the lowercase t and

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then equals new Tower followed by open and
closing parentheses.

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Remember, we end all statements
in C# with a semicolon.

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This creates a new variable called
tower with a lowercase t that

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is of type Tower with an uppercase T and
assigns it a newly created Tower object.

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In C# the convention is to name classes
starting with a capital letter.

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This is handy when creating
objects because then we

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can just name them the same as their types
except the first letter is in lowercase.

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We could have named this anything.

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We could have named it Bob except that
that wouldn't make a lot of sense.

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But we wanna give our
variables meaningful names.

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For now,
tower with a lower case t will do.

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Let’s review what we just did.

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We just used the tower cookie cutter
to stamp out a new tower cookie,

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and that's how you create an object.

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Now you'll hear me and others use the
terms object and instance interchangeably.

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By creating a tower object, we've just
created an instance of the tower class.

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We could also call this a tower instance.

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In fact, the term for creating an object
from a class is called instantiation.

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We just instantiated an object
from the tower class.

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Sometimes you might think that all of this

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redundant terminology is just there
to confuse you and maybe it is.

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But even so,

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it's good to be able to recognize these
terms when you hear or read them.

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Just remember that an object
is an instance of a class and

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you can use the terms object and
instance interchangeably.

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Let's go ahead and
create the rest of our classes.

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We'll need to have one for Invader.

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We'll create a new file called Invader.cs.

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To save on typing,
I'll just copy the code from Tower.cs.

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Paste it here and then change
the name of the class to Invader.

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Let's make another one for map in map.cs

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We'll also make one for path.

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As you can see, we've created classes for

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all of the nouns in our
description except for player.

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For now, let's hold off on
creating a class for the player.

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We now have five files in our project,
one for each of the classes we've created.

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You don't have to create
a separate file for each class.

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You also don't need to name
them the same as the class.

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This is just the convention
that most people use.

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You'll notice that a lot of
the way that we name things and

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organize things in a C# project
are based on convention.

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Conventions are not hard and set rules.

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They're just things that developers have
come to a general consensus to do in

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order to make the code easier to read and
to use.

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Using conventions helps make the code
look more professionally done.

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What we've just done is create
a scaffold for a game project.

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Now we just need to fill
out the class definitions.