Variables and Data Types

Variables and Data Types

What it means

A variable is a named storage location with a specific type, declared once and (unless const/readonly) reassignable afterward. C# is statically typed — every variable’s type is known and checked at compile time, either written explicitly (int age = 30;) or inferred with var (var age = 30;, still strictly typed as int, just written more concisely).

Examples

int age = 30;                 // explicit type
var name = "Alice";           // inferred as string -- var is NOT the same as JavaScript's `var`
double price = 19.99;
bool isActive = true;
string message = $"Hi {name}, you are {age}";   // string interpolation

const double Pi = 3.14159;    // compile-time constant, can never change
readonly DateTime Created;    // set once, typically in a constructor, then never reassigned

// Value types vs reference types
int x = 5;
int y = x;      // y gets its OWN COPY of the value
y = 10;
Console.WriteLine(x);   // 5 -- unaffected, since int is a value type

Common mistake

Assuming var makes C# dynamically typed, like JavaScript’s var or Python — var is purely a compile-time convenience; the variable’s type is still fixed and fully checked, it’s just inferred from the initializer instead of written out.

var count = 5;
// count = "five";   // Error: cannot convert 'string' to 'int' -- `count` is still strictly typed as int!

// `var` requires an initializer, since the type must be inferred from something
// var x;   // Error: implicitly-typed variables must be initialized

Quick practice

1. Is var in C# the same kind of “dynamic” typing as var in JavaScript?

   Answer

   No — C#'s var is inferred once at compile time and the variable remains strictly, statically typed afterward; it's purely a syntax convenience, not dynamic typing.

2. What’s the difference between const and readonly?

   Answer

   const values must be known at compile time and can never change; readonly fields can be assigned once at runtime (typically in a constructor) and are then fixed for the object's lifetime.

3. If int y = x; copies a value type, does changing y afterward affect x?

   Answer

   No — value types are copied by value, so x and y become completely independent after the assignment.