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In C# not everything has to be an object.

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We often have a need to create a set
of methods and properties that can

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be accessed from any class without having
an instance of a class to work with.

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This is what static classes are for.

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Static classes can't inherit
from any other class.

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Nor can static classes be inherited from.

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We also can't instantiate a static class.

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Instead of working with
objects of the class,

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we work directly with the class itself.

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An example of a static class in the .NET
framework is the System.Math class.

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System.Math is simply a group of
math functions that can be called.

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We call them directly on
the Math class itself.

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We don't need to have
a math object to use them.

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Another example of a static
class that we're familiar with

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is the system.console class.

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It makes sense for the system.console
class to be a static class because

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a program can interact with
only a single console window.

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You may have noticed that we can call
methods directly on the console class,

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without first creating a console object.

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Members in a static class are globally
accessible inside the application.

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Public fields and

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properties inside of a static class
are known as global variables.

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Typically, we want to avoid creating and
using global variables because

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we have very little control
over who can change them.

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This can cause bugs that are very
difficult to track down and resolve.

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And this is something we want to be
aware of when creating static classes.

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It's best to create static classes where
the members of the class are immutable.

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An immutable class is a class where the
data inside the class can't be changed.

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System.Math is an example
of an immutable class.

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System.Math is simply a collection
of related functions.

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There are some cases where it is all
right to have a mutable static class

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if the side effects of changing
the data in the class is manageable.

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System.console is one example.

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We can use a static class to simplify some
of the code in the Treehouse Defense game.

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Let's take a look at the Tower class.

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The Tower class contains
a static field named _random.

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We also have a static random
field in shielded invader.

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We can refactor this to use
a single static random class.

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We'll create our own static
random class in random.cs

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We'll put it in
the TreehouseDefense namespace.

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This is a good example of
why we need namespaces.

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We already have a class named
random in the system namespace.

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Because these classes are in
different namespaces,

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we can specify precisely
which one we want to use.

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Inside the class,
we'll have a static random field and

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initialize it to an instance
of the Random class.

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So, it's a private static

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Random_ random = new Random.

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We need to specify that we want
to use System.Random here,

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not the random class that we're in.

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The method in the random class
that both the Tower class and

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the shielded invader
class use is next double.

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Remember the next double
method when called,

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returns a random double between zero and
one.

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We'll create our own static next double
method that returns the result of

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calling the random objects
next double method.

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Essentially what we've done here
is created a global random object.

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Now whenever random.next double is called,

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it will get the next random number from
the global random object we have here.

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Now we can go back to all the places
that is using System.Random.

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And replace them to use
our global random class.

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So, here in the Tower class,
we'll delete this line and

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change this to our new
static Random class.

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We can do the same in shielded invader.

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Delete this line and change this, so
that it refers to the random class.

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Let's compile to make sure
we didn't break anything.

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I got a compiler error here.

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It says, cannot declare instance
members in a static class.

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Hm, that's because we didn't make
the next double a static method.

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Try it again.

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There we go, so
why would we wanna use our static class

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instead of the way we
were doing it before?

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The way we were doing it before here
in Tower and shielded invader is fine.

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But by creating a static random class,

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we've been able to slightly simplify
the code in Tower and shielded invader.

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Whenever we want a random number
between zero and one, we can just call

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random.nextdouble instead of first
creating an instance of the random class.

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Our random class is not immutable because
the state of the random object here is

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changed every time we ask for
a new random number.

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That's how it knows to give
us the next random number.

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But this is all right, since all we
care about is getting a random number.

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This is one of the rare cases where
having a globally mutable object is okay.