Variables: declare, initialize, assign

Declaration, initialization, assignment

int score;              // declaration: a slot named score that holds int
score = 10;             // assignment: put a value in
int level = 3;          // declaration + initialization in one line
score = score + 5;      // reassignment: the old value is read, then replaced

A variable's type is fixed forever at declaration. int score = "ten"; is a compile error — not a runtime surprise. That's the single biggest difference from scripting languages, and it's a feature: the compiler catches a whole class of bugs before your program ever runs.

Definite assignment: C# refuses to let you read a local variable before you've written it:

int x;
Console.WriteLine(x);   // error CS0165: use of unassigned local

The compiler tracks this per-path. It looks pedantic; it deletes an entire bug family (reading garbage memory) at zero cost.

Identifiers

Names are case-sensitive (score and Score differ), must start with a letter or _, and conventionally use camelCase for locals. Prefer names that carry meaning: elapsedMs beats e; invoiceCount beats n. The compiler doesn't care; your teammates (and you-in-six-months) do.

Multiple declarations

int a = 1, b = 2;   // legal, but usually clearer as two lines