Variables and Data Types

C# is a statically typed language — every variable has a type known at compile time. This lesson covers the type system, value types, reference types, strings, and nullable types.

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Declaring Variables

Variables in C# are declared with a type followed by a name:

int age = 30;
string name = "Alice";
double price = 19.99;
bool isActive = true;

Type Inference with var

The var keyword lets the compiler infer the type from the assigned value:

var age = 30;          // int
var name = "Alice";    // string
var price = 19.99;     // double
var isActive = true;   // bool

Tip: Use var when the type is obvious from the right-hand side. Use explicit types when clarity is needed.

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Value Types vs Reference Types

Category	Stored In	Assignment	Default	Examples
Value types	Stack	Copies the value	0, false, etc.	int, double, bool, struct, enum
Reference types	Heap	Copies the reference	null	string, class, array, object

// Value type — independent copies
int a = 10;
int b = a;
b = 20;
Console.WriteLine(a); // 10 (unchanged)

// Reference type — shared reference
int[] arr1 = { 1, 2, 3 };
int[] arr2 = arr1;
arr2[0] = 99;
Console.WriteLine(arr1[0]); // 99 (changed!)

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Numeric Types

Integer Types

Type	Size	Range
byte	8-bit	0 to 255
short	16-bit	-32,768 to 32,767
int	32-bit	-2.1 billion to 2.1 billion
long	64-bit	-9.2 quintillion to 9.2 quintillion

Each has an unsigned variant: sbyte, ushort, uint, ulong.

int count = 42;
long bigNumber = 9_000_000_000L;   // underscore separators for readability
byte flags = 0b_1010_0011;         // binary literal
int hex = 0xFF;                    // hexadecimal literal

Floating-Point Types

Type	Size	Precision	Suffix
float	32-bit	~7 digits	f
double	64-bit	~15 digits	(default)
decimal	128-bit	~28 digits	m

float temperature = 36.6f;
double pi = 3.14159265358979;
decimal salary = 75_000.50m;  // Use decimal for money!

Tip: Always use decimal for financial calculations to avoid floating-point rounding errors.

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Boolean Type

bool isLoggedIn = true;
bool hasPermission = false;

C# does not allow implicit conversion between integers and booleans (unlike C/C++).

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Characters and Strings

char