CS C#

C#: Variables & Data Types

What you will learn

How to declare variables with explicit types, how var infers the type, and the difference between value types (primitives) and reference types.

C# is statically typed — every variable has a known type at compile time.

string name = "Alice";
int age = 25;
double height = 5.6;
bool isStudent = true;

Type inference with var

var message = "Hello";   // inferred as string
var count = 42;           // inferred as int
var price = 19.99m;       // inferred as decimal (m suffix)

var tells the compiler to infer the type from the initializer. The type is still fixed — you just don't write it. Use var when the type is obvious (e.g., new List<string>()).

Common types

Type Category Size Example Notes
int value 4 bytes 42 32-bit integer
long value 8 bytes 42L 64-bit integer
double value 8 bytes 3.14 Double-precision float
decimal value 16 bytes 19.99m High precision (money)
bool value 1 byte true
char value 2 bytes 'A' Unicode character
string reference object "hello" Immutable text
arrays reference object new int[3]

Common mistakes

  • Using float when you meant double — 3.14 is a double literal by default. Use 3.14f for float.
  • Using decimal without the m suffix — 19.99 is a double; 19.99m is a decimal.
  • Forgetting that string is a reference type — but it's immutable, so it behaves like a value type in practice.
  • Using var when it obscures the type (e.g., var result = GetData() — what type is result?).

Quick check below!

Next: Control Flow