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[MUSIC]

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Hello and welcome, I'm Jeremy.

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If you've started learning C# by
taking the courses here at treehouse,

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then you've already learned a lot
about the C# programming language.

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With what you've learned so

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far, you can do a lot and I hope you had
the chance to write some code on your own.

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In the C# objects course we learned
about object oriented programming.

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If you remember, object oriented
programming makes it easier to think about

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architect in software programs
because we can think about software

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as a number of different objects
that interact with each other.

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Because object oriented programming
naturally breaks the program into types

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of objects, object oriented code
is easier to reuse and maintain.

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In this course we'll learn some slightly
more advanced concepts of object oriented

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programming and see how these
concepts are supported by C#.

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These language features and principles
are used by most object oriented programs.

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This is especially so
in software frameworks.

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Frameworks such as the .NET framework, for

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example, are libraries of code that can be
used by including them in our own code.

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Frameworks are what make
it possible to easily write

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large software programs such as websites,
games, interactive desktop software,

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mobile device applications and
so much more.

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You may find yourself writing
your own frameworks and

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libraries that'll be used by
other software developers.

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If so you'll likely be using much
of what you learn in this course.

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In any case,
knowing how these language features work

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will help you understand how to work
with existing libraries and frameworks.

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Later, when we learn more about using
the various classes provided by the .NET

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Framework, you'll be grateful you
have a good grasp on these concepts.

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You may recall that there are four
principles of object-oriented programming.

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The principles we've learned so
far are inheritance and encapsulation.

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We'll briefly review these principles
here and learn about two more,

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polymorphism and abstraction.

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These terms sound really technical but

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the concepts they represent
are actually quite straightforward.

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This course will build on top of what we
already know about C#, I recommend you

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follow the links in the teacher's notes if
you haven't completed the prerequisites.

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Or if you feel like you need to review,

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you can always do a quick refresher by
watching some of those videos again.

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We started learning about object oriented
programming in the C# Objects course where

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we wrote the code for a tower defense
game called Treehouse Defense.

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We'll use what we learn in this
course to make it easier to extend

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the Treehouse Defense game.

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To extend code means to add
capabilities to it without

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rewriting any of the original code.

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When we're done it will be possible to
add different types of invaders and

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powers to the game with minimal effort.

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This sounds like a lot of fun and
I'm excited to get going.

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Let's begin by taking a look at the code
that we ended with in C# objects