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Data Types and Variables C#


  • REPL stands for Read-Evaluate-Print-Loop. A REPL is a command line tool that allows users to enter lines of code and see the result in the terminal when executed.


  • You may launch your REPL at any time with the command $ dotnet script.
  • Commands meant to run in the REPL are preceded with a > in our curriculum. Commands meant to run in the standard command line are still preceded with $.
  • Once open, you can exit the REPL with the Ctrl + C command.

Strings and Concatenation Terminology

  • String: A sequence of characters between two quotation marks which is generally used for text content.
  • Concatenate: The act of making a new string out of multiple smaller strings, usually with a + operator, like this: "Hello" + " " + "World", which creates the string "Hello World".


Here's an example of concatenating multiple strings together in the C# REPL:

> "Programming" + " " + "is" + " " + "awesome!"
"Programming is awesome!"
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  • In C#, all lines of code must end with a semi-colon (;). There are a few exceptions we'll learn later, but don't worry about this for now.
  • We can open the REPL with the $ dotnet script command in the Terminal (for Mac) or Command Prompt (for Windows).

Integers and Arithmetic Terminology

  • Integers: A type of data representing whole numbers (numbers without decimals).
  • Operators: A special character (or characters) that indicates an action to be performed.+/, and % are all operators for mathematic functions.


Basic arithmetic works just like you'd expect:

1 + 2
4 - 3
5 * 6
9 / 2
7 + 8 * 9
(7 + 8) * 9
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  • 9 % 2 returns the remainder of 9 divided by 2. % is called modulo.


  • Assignment Operator: The = operator is used to set the initial value of a variable:
string exampleVariable = "hey, I'm a variable!";`
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  • Lower Camel Case: C# uses lower camel case for variables just like JavaScript. Here are a few examples: likeThis, or evenLikeThisExampleHere.
  • Strongly-Typed Language: A language in which data types must be declared. In the following example, we state that our variable has the string data type:
string anotherExampleVariable = "hey, I'm a variable too!";
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  • C# variables must declare the type of data they contain because C# is a strongly typed language.
  • To create a C# variable we need three things:
  • A descriptive name.
  • A data type to declare what data the variable holds such as int or string.
  • An initial value set with the assignment operator (=).

Variables and Data Types Examples

> string phrase = "Hello World";
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  • string is the data type our variable will be.
  • phrase is our descriptive name.
  • "Hello World" is the initial value.


  • Arguments: Additional information provided to a method, passed in through the parentheses following the method name.
  • Substring: A small portion of a larger string. A bit like a single word in a larger sentence. For instance, "hello" is a substring of the string "hello world".
  • Pascal Case: Required naming convention for all C# methods in which all words are capitalized with no spaces. Examples include LikeThisMethodNameHere() or ThisOtherFictionalMethodHere().

Methods Examples

Here are some examples of built-in methods you can call without arguments in the REPL:

> string phrase = "Programming is AWESOME";

> phrase
"Programming is AWESOME"

> phrase.ToUpper()

> phrase.ToLower()
"programming is awesome"
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Calling built-in methods with provided arguments:

> string phraseOne = "hello world";
> string phraseTwo = "hello";

> phraseOne.Contains(phraseTwo)
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