Parsing text to numbers

The Parse family

int i = int.Parse("42");            // 42
int neg = int.Parse("-17");         // -17
double d = double.Parse("3.5");     // 3.5
decimal m = decimal.Parse("19.99"); // 19.99
int bad = int.Parse("4 2");         // FormatException
int empty = int.Parse("");          // FormatException

Each type's Parse accepts exactly the formats its own ToString produces (plus leading/trailing spaces, which are trimmed). int.Parse("3.5") fails — there's no silent truncation on the way in. Whitespace inside the number fails; a decimal point for an int fails.

Culture matters: by default Parse uses the machine's culture — on systems configured with , as the decimal separator, double.Parse("3.5") fails. For this course's challenges the sandbox culture parses .; when you build real apps you'll meet CultureInfo.InvariantCulture for machine-to-machine formats. Knowing the trap exists is the beginner takeaway.

Choosing the type

Parse into the type that models the value: int for counts, double for measurements, decimal for money. If the input is "19.99", int.Parse is the wrong tool before you even ask whether it throws.

What a FormatException looks like

Unhandled, it kills the program with a stack trace. You'll learn try/catch in Module 15 — but for input validation the right tool comes first: TryParse, which never throws at all.