C# Variables and Data Types

Declaring Variables

int age = 25;
double price = 9.99;
bool isActive = true;
char grade = 'A';
string name = "Alice";

Built-In Types

C# Type	.NET Type	Size	Example
int	Int32	4 bytes	42
long	Int64	8 bytes	9999999999L
float	Single	4 bytes	3.14f
double	Double	8 bytes	3.14159
decimal	Decimal	16 bytes	9.99m (money)
bool	Boolean	1 byte	true / false
char	Char	2 bytes	'X'
string	String	dynamic	"hello"

var — Type Inference

var count = 10;         // int
var price = 9.99;       // double
var name = "Alice";     // string

// Useful in loops:
var numbers = new List<int> { 1, 2, 3 };
foreach (var n in numbers) Console.WriteLine(n);

Constants and readonly

const double PI = 3.14159265;   // compile-time constant
readonly int maxSize;            // set once in constructor

// PI = 3.0; // ❌ error

Nullable Types

int? nullableInt = null;
string? nullableStr = null;

// Null-coalescing operator
int value = nullableInt ?? 0;          // 0 if null

// Null-conditional operator
int? len = nullableStr?.Length;        // null if str is null

String Interpolation

string firstName = "Alice";
int age = 30;
string msg = $"My name is {firstName} and I am {age} years old.";
Console.WriteLine(msg);