Chapter 1:
C#: Variables, Data Types, and Collections

Objectives

  • Define variables, constants, and data types in C#
  • Distinguish between common data types (integers, floats, booleans, characters, strings)
  • Apply type inference using the var keyword
  • Create and manipulate arrays, lists, and dictionaries
  • Utilize modern C# collection expressions and target-typed new syntax
  • Implement raw string literals for multi-line text

Table of Contents

Comments

A comment is a special type of code that is ignored by computers. Comments are written as notes for other programmers or as reminders. They are part of the code, but not run when a program does.

In C#, a single-line comment start with two backslashes, //. Anything after the backslashes until the end of the line is considered a comment.

Example:

// Single line comment!
// Another single line comment!

For multi-line comments, a single backslash, /, and asterisk, *, mark the beginning of a multiline comment. It ends when a asterisk, *, followed by a backslash, /.

/* This is a
multi-line
comment. */

Variables and Constants

A variable is a container for data that can change when a program is run. Depending on the type of data, different labels, data types, are used when the container is created to explain how the data should be treated or stored when a program is run.

example = 5;

Variable names can contain letters, numbers, and the underscore. They cannot contain any spaces.

The following are valid variables names:

example = 5;
example2 = 6;
_example = 7;
anotherExample = 8;

Note: Because variable names cannot contain spaces, it is very common to write variable names in a pattern using uppercase and lowercase letters to mark where the next word starts.

Common Data Types

There are many different data types. These identify the type of data in the variable. They also appear before the name of the variable when it is created.

Some of the more common types are Integers, Floats, Booleans, and Characters.

Integers

An Integer is a whole number. It does not contain a decimal point. Examples include numbers like 5, 15, and 899999.

An Integer can be any number within the range of -2,147,483,648 to 2,147,483,647.

Integers are created in C# through the keyword int.

int example = 5;

Floats

A Float is a value containing a decimal point. They are named “floats” based on the term “floating point,” which is the technical name for any decimal point values represented digitally. They are called this because decimal point can “float.”

Float value can be between -3.4 x 1038 to +3.4 x 1038.

Floats are created through the keyword float.

float example = 3.14f;

Note: The use of the letter f after a Float value is part of how decimal numbers are written. It lets C# know not to convert the value into something else and leave it a “float” value.

Booleans

A Boolean value is named after George Boole, who invented a way to write algebra using only true (1) and false (0) values.

Boolean values can be either true or false.

They are created using the keyword bool.

bool example = true;

Note: A Boolean variable can only use the special keywords true and false for its values.

Characters

A Character is a single letter, number, or other symbol enclosed in single quotation marks.

Characters are created through the keyword char.

char example = '3';

Note: Characters must be a single letter, number, or other symbol. Their values also must be enclosed in single quotation marks.

Constants

Variables change. However, there is a special type of variable called a constant that does not. In C#, a constant, once created, cannot ever change its value.

Constants are created using the keyword const. It appears before the label for any other data type.

const float PI = 3.14159f;

Note: A common pattern when creating constants is to write it using only capital letters. This signals its visual importance in the code.

Type Inference with var

Modern C# (since version 3.0, enhanced in later versions) supports type inference using the keyword var. When a variable is declared with var, the compiler automatically determines the type based on the value assigned to it.

var example = 5;          // Inferred as int
var pi = 3.14f;           // Inferred as float
var greeting = "Hello!";  // Inferred as string
var isActive = true;      // Inferred as bool

Note: While var is convenient and widely used in modern C#, it can only be used when the type can be clearly inferred from the assignment. The variable is still strongly typed - it doesn’t change the type, just lets the compiler figure it out for you.

Collections

Many programming languages define types of data that can hold others. In C#, these are known as collections because they “collect” other data types.

Arrays

The most common type of collection in any programming language is an Array. It defines a sequence of values where order is important.

int[] exampleArray;

Arrays are created using the name of a data type and then opening ([]) and closing (]) square brackets after it. They appear before the name of the variable.

Values are created with an Array in C# through specifying its data type, length (number of entries), and then its initial values in opening ({) and closing (}`) curly brackets.

int[] example = new int[5]{1,2,3,4,6};

Note: The new keyword signals that an object is being created. Classes and objects are explained in the chapter on Object-Oriented Programming.

Modern C# (9.0+) also supports target-typed new expressions, which lets you omit the type on the right side when it’s obvious:

int[] example = new[]{1,2,3,4,6};      // Simpler syntax
int[] example2 = [1,2,3,4,6];          // Collection expression (C# 12.0+)

Values within an array are defined in reference to their position called their index. To access data within an array, its index is needed. In C#, indices start with 0 and increase up to (1 - length).

int[] example = new int[5]{1,2,3,4,6};
// 2
example[1];
// 1
example[0]

Lists

A List is like an Array, but provides the ability to search, sort, and manipulate its entries. Order is not important to a List.

Lists are created using a notation of <Type> where each “Type” is a known data type following the keyword List.

List<char> exampleList = new List<char>();

In modern C#, you can use simpler syntax:

var exampleList = new List<char>();    // Target-typed new
List<char> exampleList2 = [];         // Collection expression (C# 12.0+)

Values are accessed through using opening ([) and closing (]) square brackets for their index.

List<char> exampleList = new List<char>();
exampleList.Add('A');
exampleList.Add('B');
// 'A'
exampleList[0];

Dictionaries

A Dictionary is a set of key-value pairs. A key is used to refer to a value. A dictionary is named as much because, like its namesake, something is looked up (a key) to get something else (a value).

Dictionaries are created using a notation of <Type, Type> where each “Type” is a known data type following the keyword Dictionary.

Dictionary<int, char> dict = new Dictionary<int, char>();

In modern C#, you can use simpler syntax:

var dict = new Dictionary<int, char>();  // Target-typed new

In the above example, the key would be an Integer and any values associated with it Characters.

Values are accessed through using opening ([) and closing (]) square brackets for their key.

Dictionary<int, char> dict = new Dictionary<int, char>();
dict.Add(99, 'A');
// 'A'
dict[99];

Note: In order to be accessed by their key, values must be added to a dictionary through the method Add(). Methods are explained as part of the Object-Oriented Programming chapter.

Strings

A String is a special type of collection that can only contain Characters.

Strings are created using the keyword string

string example = "Hi!";

Note: String values are always enclosed in double-quotation marks.

Modern C# (11.0+) also supports raw string literals using triple quotes, which is especially useful for multiline strings or strings containing quotes:

string json = """
    {
        "name": "Fred",
        "age": 25
    }
    """;