Classes & Objects

var ana = new BankAccount("Ana");
var ben = new BankAccount("Ben");

ana.Deposit(100);
ana.Deposit(50);
ben.Deposit(20);

Console.WriteLine(ana.Describe());
Console.WriteLine(ben.Describe());

class BankAccount
{
    private string owner;          // fields: each object has its own
    private decimal balance;

    public BankAccount(string owner)
    {
        this.owner = owner;         // this.owner is the field, owner the parameter
    }

    public void Deposit(decimal amount)
    {
        balance += amount;          // same as this.balance += amount
    }

    public string Describe() => $"{owner}: {balance:0.00}";
}

var t = new Thermostat();
t.Celsius = 21.5;
Console.WriteLine($"{t.Celsius} C = {t.Fahrenheit} F");

t.Celsius = -500;                     // rejected by the setter
Console.WriteLine($"still {t.Celsius} C");
Console.WriteLine($"changed {t.Changes} times");

class Thermostat
{
    private double celsius = 20;     // backing field

    public double Celsius
    {
        get => celsius;
        set
        {
            if (value < -273.15) { Console.WriteLine($"ignored {value}"); return; }
            celsius = value;
            Changes++;
        }
    }

    public double Fahrenheit => Celsius * 9 / 5 + 32;   // computed, read-only

    public int Changes { get; private set; }            // auto-property, private setter
}

var order = new Order { Id = 42, Customer = "Ana", Note = "leave at door" };
Console.WriteLine($"#{order.Id} for {order.Customer}: {order.Note}");

var bare = new Order { Id = 43, Customer = "Ben" };   // Note is optional
Console.WriteLine($"#{bare.Id} note is empty: {bare.Note == ""}");

class Order
{
    public required int Id { get; init; }
    public required string Customer { get; init; }
    public string Note { get; init; } = "";
}

var a = new Rectangle(3, 4);
var b = new Rectangle(5);            // a square, via : this(side, side)
var c = new Rectangle();             // default 1 x 1

foreach (var r in new[] { a, b, c })
    Console.WriteLine($"{r.Width} x {r.Height} = {r.Area}");

class Rectangle
{
    public int Width { get; }
    public int Height { get; }
    public int Area => Width * Height;

    public Rectangle(int width, int height)
    {
        if (width <= 0 || height <= 0)
            throw new ArgumentException("sides must be positive");
        Width = width;
        Height = height;
    }

    public Rectangle(int side) : this(side, side) { }

    public Rectangle() : this(1) { }
}

var hi = new Greeter("Hello");
Console.WriteLine(hi.Greet("Ana"));
Console.WriteLine(hi.Greet("Ben"));
Console.WriteLine(hi.Count);

var card = new Card { Title = "Intro", Pages = 3 };
Console.WriteLine($"{card.Title} ({card.Pages} pages)");

class Greeter(string greeting)       // primary constructor
{
    public int Count { get; private set; }

    public string Greet(string name)
    {
        Count++;
        return $"{greeting}, {name}!";   // parameter captured by the class
    }
}

class Card
{
    public string Title { get; set; } = "Untitled";
    public int Pages { get; set; }
}

var t1 = new Ticket("concert");
var t2 = new Ticket("museum");
var t3 = new Ticket("concert");

Console.WriteLine($"{t1.Number} {t2.Number} {t3.Number}");
Console.WriteLine($"issued: {Ticket.Issued}");
Console.WriteLine(Units.KmToMiles(42.195).ToString("0.00"));

class Ticket
{
    public static int Issued { get; private set; }   // one shared counter

    public string Number { get; }

    public Ticket(string eventName)
    {
        Issued++;
        Number = $"{eventName[..3].ToUpper()}-{Issued:000}";
    }
}

static class Units
{
    public const double MilesPerKm = 0.621371;
    public static double KmToMiles(double km) => km * MilesPerKm;
}

var a = new Money(10m, "USD");
var b = new Money(10m, "USD");
Console.WriteLine(a);
Console.WriteLine($"records equal: {a == b}");

var c1 = new MoneyClass(10m, "USD");
var c2 = new MoneyClass(10m, "USD");
Console.WriteLine($"classes equal: {c1 == c2}");

var raised = a with { Amount = 15m };   // copy with one property changed
Console.WriteLine($"{a.Amount} -> {raised.Amount} {raised.Currency}");

var (amount, currency) = raised;         // deconstruction for free
Console.WriteLine($"{amount} {currency}");

record Money(decimal Amount, string Currency);

class MoneyClass(decimal amount, string currency)
{
    public decimal Amount { get; } = amount;
    public string Currency { get; } = currency;
}

var points = new List<PointS> { new PointS(1, 2) };
var first = points[0];            // a COPY of the struct
first.X = 50;
Console.WriteLine(points[0].X);   // the list is unchanged

points[0] = first;                // write the copy back
Console.WriteLine(points[0].X);

PointS origin = default;          // all fields zero, never null
Console.WriteLine($"({origin.X}, {origin.Y})");

struct PointS(int x, int y)
{
    public int X = x;
    public int Y = y;
}

var a = new Isbn("978-0134494166");
var b = new Isbn("9780134494166");      // same book, no dashes

Console.WriteLine(a);
Console.WriteLine($"Equals: {a.Equals(b)}");

var shelf = new HashSet<Isbn> { a };
Console.WriteLine($"shelf has b: {shelf.Contains(b)}");

class Isbn
{
    public string Digits { get; }

    public Isbn(string text) => Digits = text.Replace("-", "");

    public override string ToString() => $"ISBN {Digits}";

    public override bool Equals(object? obj) =>
        obj is Isbn other && other.Digits == Digits;

    public override int GetHashCode() => Digits.GetHashCode();
}

string? nickname = FindNickname("ben");

// Three safe ways to use a string? value
if (nickname is not null)
    Console.WriteLine($"length {nickname.Length}");

int? length = nickname?.Length;                                  // ?. short-circuits to null
Console.WriteLine($"has a length: {length.HasValue}");
Console.WriteLine($"shown as: {nickname ?? "(none)"}");       // ?? supplies a fallback

var ana = new User("Ana", FindNickname("ana"));
Console.WriteLine(ana.Display);

static string? FindNickname(string user) => user == "ana" ? "Annie" : null;

class User(string name, string? nickname)
{
    public string Display => nickname is null ? name : $"{name} ({nickname})";
}