📜 WAYPOINT LESSON

Modeling the domain

⭐ beginner⏳ 13 min read📍 Lesson 83 of 85

A record for Transaction, an enum for direction, validation that returns reasons — a small domain model done right.

The types

public enum Direction { Income, Expense }

public record Transaction(int Id, Direction Dir, decimal Amount,
                          string Category, string Description, DateTime Date);

public record AddResult(bool Ok, string Error);

The record gives value equality for free (Module 11); the enum makes invalid directions unrepresentable — Direction.Expense beats a magic "out" string the compiler can't check. decimal for money (Module 2): exact base-10 arithmetic, no binary-rounding surprises.

Validation as data

AddResult returns why an entry was rejected instead of throwing: Ok = false, Error = "amount-must-be-positive". Callers branch on the result; no exception cost, no control flow by crash. The error strings are the API — tests pin them.