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📜 WAYPOINT LESSON

Checkpoint — Classes

⭐ beginner⏳ 20 min read📍 Lesson 37 of 85

A bank account: an owner, a balance, and two doors — every mutation validated, every invariant testable.

Checkpoint: the bank account

Task: implement nested class Solution.BankAccount:

  1. Constructor (string owner, decimal openingBalance) — throws ArgumentException for a null/whitespace owner or a negative opening balance.
  2. Read-only Owner and Balance properties.
  3. Deposit(decimal amount) — throws ArgumentException unless amount > 0; otherwise adds to the balance.
  4. Withdraw(decimal amount) — throws ArgumentException unless amount > 0, throws InvalidOperationException when funds are insufficient; otherwise subtracts.

The invariant: the balance only ever changes through positive, funded operations. A rejected operation must leave the balance exactly as it was.