A class can protect a rule, not just hold fields

First, build a mental model

An invariant is a statement that remains true before and after every valid operation. Encapsulation narrows the ways callers can change the state so the class can defend it.

1. A constructor establishes a valid starting state or rejects construction.
2. A public getter with a private setter lets callers observe state without assigning it directly.
3. Methods validate a requested transition before changing any fields.
4. A class invariant protects one object in this process. Database constraints and transactions are still needed for concurrent persistent updates.

var stock = new StockItem(5);
Console.WriteLine(stock.TryReserve(2));
Console.WriteLine(stock.TryReserve(4));
Console.WriteLine(stock.Available);

sealed class StockItem
{
    public int Available { get; private set; }
    public StockItem(int initial)
    {
        if (initial < 0) throw new ArgumentOutOfRangeException(nameof(initial));
        Available = initial;
    }
    public bool TryReserve(int quantity)
    {
        if (quantity <= 0 || quantity > Available) return false;
        Available -= quantity;
        return true;
    }
}

1. Construct

   The initial valid stock is 5.

2. Reserve 2

   The request passes; Available becomes 3.

3. Reserve 4

   The request fails before mutation; Available stays 3.