Properties

// ===== Auto-properties =====
public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
}

// ===== Init-only (immutable after construction) =====
public class User2
{
    public int Id { get; init; }
    public string Name { get; init; } = string.Empty;
    public string Email { get; init; } = string.Empty;
}

var u = new User2 { Id = 1, Name = "Alex", Email = "a@x.io" };
// u.Name = "Sam";   // error: init-only

// ===== Records (the modern shape for DTOs) =====
public record Order(int Id, string Customer, long TotalCents);

var o = new Order(1, "Alex", 4995);
var o2 = o with { TotalCents = 5000 };

// ===== Read-only properties =====
public class Counter
{
    private int _count;
    public int Count => _count;            // expression-bodied getter
    public void Increment() => _count++;
}

// ===== Computed (no backing field) =====
public class Order2
{
    public long TotalCents { get; init; }
    public decimal TotalAud => TotalCents / 100m;
}

// ===== Custom getter / setter =====
public class User3
{
    private string _email = string.Empty;
    public string Email
    {
        get => _email;
        set
        {
            if (!value.Contains('@')) throw new ArgumentException("invalid email");
            _email = value;
        }
    }
}

// ===== Private setter (visible only in class) =====
public class Cart
{
    public List<string> Items { get; private set; } = new();
    public void Add(string sku) => Items.Add(sku);
}

// ===== Static properties =====
public static class Config
{
    public static string AppName { get; } = "shop";
    public static int MaxRetries { get; set; } = 3;
}

// ===== Required (C# 11+) =====
public class Settings
{
    public required string ApiKey { get; init; }
    public string Region { get; init; } = "ap-southeast-2";
}

var s = new Settings { ApiKey = "..." };
// var bad = new Settings();   // error: ApiKey not initialised

// ===== Indexers (special properties) =====
public class Cache
{
    private readonly Dictionary<string, string> _data = new();
    public string this[string key]
    {
        get => _data[key];
        set => _data[key] = value;
    }
}

var c = new Cache();
c["name"] = "Alex";
var n = c["name"];

// ===== Pattern: invariants in constructor + init-only properties =====
public class Money
{
    public long Cents { get; }
    public string Currency { get; }
    public Money(long cents, string currency = "AUD")
    {
        if (cents < 0) throw new ArgumentOutOfRangeException(nameof(cents));
        Cents = cents; Currency = currency;
    }
}

// ===== Patterns to internalise =====
// - init-only properties for immutable DTOs
// - records for value-shape classes; with-expressions for non-destructive update
// - Auto-properties for trivial data; custom setter only when validation needed
// - 'required' for fields the caller MUST initialise

// ===== Pitfalls =====
// - public setter on a list property -> consumers replace the whole list
// - get-only without backing field -> recompute each access (memoise if expensive)
// - init vs set -> init is one-shot at construction; set is general
// - Forgetting nullable annotations on properties -> NREs in callers

public class Person {
    public string Name { get; set; }                  // auto-property
    public int Age   { get; private set; }            // setter restricted
    public string Initial => Name.Substring(0, 1);    // computed
}