20. Properties, indexers, static and const vs readonly

public class Account
{
    private decimal _balance;
    public decimal Balance { get => _balance; private set => _balance = value >= 0 ? value : throw new ArgumentException(); }
    public string Owner { get; init; } = "";          // settable only during initialisation
    public static int Count { get; private set; }      // shared by all instances
    public string Tier { get; set { field = value.Trim(); } } = ""; // C# 14 'field' keyword
    public decimal this[int month] => _history[month]; // indexer
    private readonly decimal[] _history = new decimal[12];
}