Variables and Constants in C#

In C#, variables and constants are the foundation for storing and manipulating data within a program.
The main difference between the two is that variables can change their value during execution, while constants remain fixed.

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Variable Declaration

A variable is a memory space that stores a value that can be modified.
It is declared by indicating the data type followed by the variable name:

int age = 25;        // Integer variable
string name = "Ana"; // Text variable
bool isActive = true; // Boolean variable

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Constant Declaration

A constant is defined with the keyword const. Its value is assigned at the time of declaration and cannot change during execution.

const double PI = 3.1416;
const string Country = "Colombia";

warning

If you try to modify a constant, the compiler will throw an error.

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Naming Conventions

In C#, it is recommended to follow good practices:

• Variables → camelCase notation: myVariable, userCounter

• Constants → UPPERCASE with underscores: MAX_VALUE, PI

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Key Differences

Feature	Variable	Constant
Value	Can change	Cannot change
Keyword	None special	const
Example	int age = 20;	const double PI = 3.1416;

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Printing Formats and Comments in C#

In C#, it is very common to show information to the user through the console, as well as documenting code with comments.
These two elements are key to writing clear and easy-to-understand programs.

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Printing Formats in C#

Printing in the console is done with Console.WriteLine() or Console.Write().

• Console.WriteLine(): prints the text and adds a line break at the end.
• Console.Write(): prints without a line break.

Basic Examples:

Console.WriteLine("Hello, world!");
Console.Write("This is printed on the same line.");

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String Interpolation

A modern and clear way to display values inside text:

int age = 20;
string name = "Ana";

Console.WriteLine($"My name is {name} and I am {age} years old.");

By using $"" you can insert variables directly into the string with { }

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Placeholders (Format Indices)

Another way to print values is using indices:

string product = "Laptop";
double price = 1500.50;

Console.WriteLine("The product {0} costs ${1}", product, price);

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Numeric Formats

C# allows applying formats to numbers:

double price = 1234.56;

Console.WriteLine("Standard price: {0}", price);
Console.WriteLine("Price with 2 decimals: {0:F2}", price);
Console.WriteLine("Currency price: {0:C}", price);

• F2: 2 decimal places

• C: Currency format

• P: Percentage

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Comments in C#

Comments serve to document code and are not executed.

Types of Comments:

• Single Line

// This is a single-line comment

• Multi-line

/* This is a comment
   spanning multiple lines */

• XML Documentation Comments Used to document methods and classes.

/// <summary>
/// Calculates the area of a rectangle
/// </summary>
public int CalculateArea(int width, int height) {
    return width * height;
}

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Data Types in C#

In C#, data types define what kind of information a variable can store.
The most common ones to start with are:

• Text strings (string)
• Numbers (int, double, float, decimal)
• Logical values (bool)

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String (Text Strings)

A string is used to store text.

string name = "Ana";
string greeting = "Hello, " + name;

Console.WriteLine(greeting); // Prints: Hello, Ana

info

• They are written inside double quotes "".

• You can concatenate (+) or interpolate ($"") strings.

Example with interpolation:

Console.WriteLine($"Welcome {name}");

Numbers (Numeric Values)

There are different types of numbers in C#:

• int → whole numbers (positive or negative).

• double → decimal numbers with high precision.

• float → less precise decimals, requiring an f at the end.

• decimal → used in financial calculations due to its high precision.

Example:

int age = 25;
double price = 199.99;
float height = 1.75f;
decimal salary = 1500.75m;

Console.WriteLine($"Age: {age}, Price: {price}, Height: {height}, Salary: {salary}");

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Boolean (Logical Values)

A bool can only hold two values:

• true

• false

It is heavily used in conditions.

bool isOfAge = true;
bool hasLicense = false;

Console.WriteLine($"Is of age: {isOfAge}");
Console.WriteLine($"Has license: {hasLicense}");

In conditionals:

if (isOfAge) {
    Console.WriteLine("You can vote.");
} else {
    Console.WriteLine("You cannot vote.");
}