3.2.10 - Local variables and modular design (C#)

Lifetime and scope of local variables

A local variable belongs to the subroutine or block in which it is declared, rather than being accessible throughout the program.

static double CalculateCost(int quantity, double unitPrice)
{
    double subtotal = quantity * unitPrice;
    return subtotal;
}

subtotal is declared inside the method. For ordinary calls, it exists while that call executes and is accessible only within its enclosing block. Code in Main cannot read subtotal directly; the returned value carries the result out. Two separate calls calculate their own subtotal. A previous call's temporary value is not a shared running total.

Local variables reduce accidental interference: another module cannot silently alter this temporary value. They also make a module's behaviour easier to understand because its data is close to the code using it. Avoid relying on hidden shared state when ordinary parameters and return values express the intended interface.

Clear documented interfaces

A module's interface describes how other code supplies its inputs and receives its output without depending on its internal temporary variables.

For CalculateCost, document the parameter order, types, meaning and permitted inputs, plus the returned value: