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Declaring variables using the var keyword,
only works if you initialize the variable

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with a value,
that is assign a value to it immediately.

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If you don't,
you'll get a syntax error saying,

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implicitly-typed variables
must be initialized.

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So what does it mean,
implicitly-typed variables?

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Values in C# are classified into
different types that specify

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what the values can be used for.

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You can view of a value by
calling the GetType method on it.

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We haven't covered methods yet.

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But for now, all you need to know is that
a method is a collection of code which you

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can call to get it to do something.

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You call a method by putting a dot,
that's the period character,

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the method name, and
a pair of parentheses after a value.

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For whole numbers, the GetType
method gives a type of System.Int32.

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For strings of text
surrounded by double quotes,

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GetType gives a type of System.String.

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C# has dozens of types built in.

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Integers, as we mentioned,
have a type of System.Int32 by default.

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Floating point numbers, that is,
numbers with a decimal point,

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have a type of System.Double by default.

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The special values true and
false has a type of System.Boolean.

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Strings of text characters
surrounded by double quotes,

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as we mentioned,
have a type of System.String.

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And individual characters of text
surrounded by single quotes have a type of

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System.Char.

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As we mentioned, types specify
what a value can be used for.

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To understand this, think about
objects in the world around you.

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You can read a book or a magazine
because those are printed materials.

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You can drink coffee or
soda because those are beverages.

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But if you try to drink a book or
read coffee, it won't go very well.

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Likewise, the type of a C# value
specifies what you can do with it.

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You can do math with numbers or
you can capitalize text.

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Likewise, the type of a C# value
specifies what you can do with it,

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you can do math with numbers or
you can capitalize text.

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But it does not make any sense
to try to capitalize the number.

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Nor does it make sense to
do math with a string.

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You'll get errors if you try.

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Those errors are actually a good thing.

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C# detects that you made a mistake,
and is letting you know about it, so

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you can fix the problem before
users of your program see it.

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And the only reason C# can detect these
kinds of errors is because of types.

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Types help protect you from
making mistakes in your code.

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In software development,
this is known as type safety.

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The assurance that when you have
a value of a particular type,

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you can do particular operations on it and
be certain those operations will work.

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Type safety is an important
feature of the C# language.

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So now let's look again at that error we
got when we declared the variable with

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the VAR keyword but
didn't assign it a value.

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It says implicitly type
variables must be initialized.

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Every variable in C# needs to have a type.

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When you use the VAR keyword C# will look
the type in the value your initialized

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in the variable with, and
use that as the type of the variable.

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That's what it means by implicitly typed,

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the initial value implies that
variable should have a certain type.

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But if you don't provide
an initial value there's no way to

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determine what the variables type should
be, and that's when you'll get errors.

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So how do you declare a variable
without assigning an initial value?

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You give it an explicit type,

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you specify what type of value
the variable should hold.

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Just write the type name
followed by the variable name and

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the variable is declared.

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Then you can assign a value to
the variable later on when you're ready

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to use it.

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Let me try saving this and running it.

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And you can see everything is working.

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If you want, you can initialize a variable
with an explicit type declaration

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just like you can when you’re
using the VAR keyword.

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So let me edit these
four statements together.

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I'll delete my number = 10 assignment and
say instead System.Int32 number = 10.

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And I'll delete my what's
up greeting assignment, And

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instead, initialize the greeting
variable with that value.

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Let me save that, try rerunning it.

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And I get the same result.

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By the way, we did something
a little unusual with the types for

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our variables that C#
developers don't normally do.

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When you call the GetType method on those
signatures, you get System.Int32 back.

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For strings,
you GetType return System.String.

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Let me run this real quick so
we can confirm that.

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Oops I forgot to say first,
let me say that.

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And go back to the console and
run it again.

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There we go System.Int32 and
System.String.

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So to make the connection
clear we use System.Int32 and

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System.String is the types
of our variables.

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But typing System.Int32 and System.String
all the time would get really tedious.

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So C# offers some shortcuts.

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A type of int with a lower case
i is equivalent to System.Int32.

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And a type of string with a lower case
s is equivalent to System.String.

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Let me save this, and
let's try running that.

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And you'll see we get the same result.

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Int converts to System.Int32 and
string converts to System.String.

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There are shortcuts for
other frequently used types too.

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Here's a program that uses variable
declarations with explicit types for

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the same integer, floating point number,
Boolean value, string and

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character value that we saw earlier.

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You can see that we use
types of System.Int32,

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System.Double, System.Boolen,
System.String and System.Char.

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Each one of those types has
an equivalent shortcut type name.

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I can replace the type in each variable
declaration with it's shortcut name, and

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the program will still work the same way.

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So System.Int32 as we already demonstrated
can be replace with lowercase int.

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System.Double can be replace
with lowercase double.

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System.Boolean can be
replaced with just bool.

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System.String can be replaced by
the single word string in lowercase.

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And System.Char can be replaced with
the abbreviation char in lowercase.

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Let me save this and run it.

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And you can see it still works and it
prints the full names out for each type.

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C# developers almost never
use the full type names for

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these built in types in their programs,
they use shortcuts instead.

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In your code you should use
lowercase shortcut names like int,

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double, bool, string, and char.

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But don't be surprised when the GetType
method returns the type System.Int32,

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System.Double, System.Boolean,
System.String, and System.Char.