Types and variables

What is a variable?

A variable is a name for a place in memory that holds a value. You declare it by writing its type and its name, and usually give it a first value. Giving the first value is called initialising.

int age = 21;
string name = "Asha";
age = 22;              // change the value
Console.WriteLine($"{name} is {age}");

Static typing: why types matter

C# is statically typed. That means every variable's type is fixed when you write the code, and the compiler checks every use. If you try to put text into an int, the build fails before anyone runs the program. This catches many bugs early, which is one reason companies like C# for large systems.

int count = 10;
count = "ten";   // error CS0029: Cannot implicitly convert type 'string' to 'int'

The common built-in types

double versus decimal: money needs decimal

double stores numbers in binary (base 2). Many simple decimals such as 0.1 cannot be stored exactly in binary, just as 1/3 cannot be written exactly in decimal. Tiny errors appear. decimal stores numbers in base 10, so 0.1 is exact. For rupees, GST and bank balances, always use decimal.

double a = 0.1 + 0.2;
decimal b = 0.1m + 0.2m;
Console.WriteLine(a);
Console.WriteLine(b);

var: let the compiler infer the type

var tells the compiler: "work out the type from the value on the right". The type is still fixed forever; var is not like JavaScript's let. You must give a value on the same line.

var total = 450;        // int
var city = "Kochi";     // string
var rate = 0.18m;       // decimal
// var x;               // error: implicitly-typed variables must be initialized
// total = "big";       // error: total is an int

const: values that never change

const decimal GstRate = 0.18m;
const int DaysInWeek = 7;
// GstRate = 0.12m;     // error: cannot assign to a const

Names and rules

• Names may use letters, digits and _, but cannot start with a digit: total2 is fine, 2total is not.
• Names are case-sensitive: total and Total are different variables.
• Local variables use camelCase (monthlyRent). Types, methods and constants use PascalCase (GstRate).
• You cannot use a variable before it has a value: error CS0165, use of unassigned local variable.

Common mistakes and fixes

• Using double for money: switch to decimal and add m to literals.
• Using single quotes for text: 'Hello' is an error; use "Hello". Single quotes are only for one char.
• Declaring the same name twice in one block: error CS0128. Reuse the variable without the type: count = 5;
• Expecting int to hold 5 billion: it overflows. Use long for large counts like phone numbers stored as numbers (better still, store phone numbers as strings).