Decimal

Decimal values and the meaning of a price

A numeric literal has a type before the program performs arithmetic. In this example, the m suffix makes 12.50m and 1.25m decimal values. Consequently, var infers decimal for both variables; it does not choose a vague, dynamically changing number type. The addition also produces a decimal. Reading those types is the first step toward understanding the calculation, because type choices determine precision, available operations, and which combinations the compiler permits.

Decimal represents base-ten quantities with a finite number of digits and an associated scale. It is commonly appropriate for prices and amounts that follow decimal business rules. It does not make every mathematical operation exact, provide unlimited range, or decide a currency's rounding policy. Division can still require rounding, and an excessive result can still overflow. Choose a numeric type according to the domain, then separately define validation, rounding, and display rules instead of expecting one suffix to solve every money problem.

Predict the one visible result

The first assignment stores a price of twelve and a half units. The second stores an extra charge of one and a quarter units. Console.WriteLine evaluates price + extra before printing anything. Align the decimal positions by hand: 12.50 + 1.25 is 13.75. Neither variable changes during that expression. The original sample therefore prints one line, 13.75, with a final line terminator supplied by WriteLine; it does not print the individual inputs first.

SharpRune's browser runtime uses invariant culture for these prepared examples, so the decimal separator in the displayed result is a period. A standalone application on another machine can format an unqualified decimal using its current culture. That is a presentation concern, distinct from how the decimal addition works. When writing a portable machine report, choose an explicit format provider. When formatting a human-facing amount, choose the intended user's culture and currency policy deliberately rather than copying the sample's console presentation blindly.

var price = 12.50m;
var extra = 1.25m;
Console.WriteLine(price + extra);

Representation does not replace a financial policy

A bare fractional literal such as 1.25 is a double, whereas 1.25m is a decimal. Mixing those types in one addition is not an implicit decimal promotion; the compiler requires a deliberate conversion. Do not add casts merely to silence the error without deciding which representation the calculation should use. Likewise, showing two fractional digits is not the same operation as rounding an amount for settlement. A formatting string can change the printed text while leaving the stored value untouched.

Two changes to calculate before running

First change only extra to 0.75m. Predict the result, then run: the total should be 13.25. Add a second WriteLine that prints price and verify that it remains the original 12.50 value in this runtime. Hint: addition returns a new numeric result; assigning a variable is a separate statement. If your output includes an unexpected additional line, inspect the number and order of WriteLine calls before changing the arithmetic.

Next replace the expression with (price + extra) * 2m while retaining the changed extra. The expected total is 26.50. Compare that with price + extra * 2m, whose multiplication occurs first and therefore gives 14.00. Hint: write intermediate totals on paper and use parentheses to express the intended calculation. Finally explain where an invoice would round its amount and how you would test that rule with a midpoint value rather than only whole-number prices.

A review before moving on

Explain the program without describing every decimal as simply money. Identify each variable's inferred type, the type of the addition, the exact original result, and the role of output culture. Then describe one calculation that still needs an explicit rounding policy. The aim is to connect a type choice to a concrete requirement while keeping representation, arithmetic, validation, and presentation as separate decisions that can each be checked.

• Both original fractional literals have the m suffix.
• The unchanged sample prints one line: 13.75.
• Neither input variable is modified by the addition.
• A display format does not by itself define settlement rounding.