Records — C# Record Types

S10.0 What This Teaches

• Record class vs plain class: value equality by default
• Positional records and primary constructors
• Non-destructive mutation with with expressions
• init-only properties
• Record structs for stack-allocated value types
• Inheritance and pattern matching with records

S10.1 Record Types

// Records - value equality on reference types.

record Person(string Name, int Age);

var alice1 = new Person("Alice", 30);
var alice2 = new Person("Alice", 30);
var bob    = new Person("Bob",   25);

Console.WriteLine(alice1 == alice2);   // True  - same values
Console.WriteLine(alice1 == bob);      // False
Console.WriteLine(alice1);             // Person { Name = Alice, Age = 30 }

S10.2 Positional Records

record Point(double X, double Y)
{
    // Computed property - not part of equality
    public double Length => Math.Sqrt(X * X + Y * Y);

    // Additional constructor
    public Point(double v) : this(v, v) { }
}

var p = new Point(3, 4);
Console.WriteLine(p.Length);   // 5

// Deconstruct into variables
var (x, y) = p;
Console.WriteLine($"x={x}, y={y}");

S10.3 Non-Destructive Mutation with with

record Address(string Street, string City, string Zip);

var home = new Address("123 Main St", "Springfield", "62701");

// Change only the city; everything else is copied
var relocated = home with { City = "Shelbyville", Zip = "62702" };

Console.WriteLine(home);       // Address { Street = 123 Main St, City = Springfield, Zip = 62701 }
Console.WriteLine(relocated);  // Address { Street = 123 Main St, City = Shelbyville, Zip = 62702 }
Console.WriteLine(home == relocated);  // False

S10.4 init-only Properties

record Config
{
    public string Host { get; init; } = "localhost";
    public int    Port { get; init; } = 8080;
    public bool   Tls  { get; init; } = false;
}

var cfg = new Config { Host = "prod.example.com", Port = 443, Tls = true };
Console.WriteLine(cfg);

// cfg.Port = 9090;   // compile error - init-only after construction

// Non-positional records still support with
var dev = cfg with { Host = "dev.example.com", Tls = false };

S10.5 Record Structs

record struct Vector2(double X, double Y)
{
    public static Vector2 operator +(Vector2 a, Vector2 b) => new(a.X + b.X, a.Y + b.Y);
    public double Length => Math.Sqrt(X * X + Y * Y);
}

var v1 = new Vector2(1, 2);
var v2 = new Vector2(3, 4);
Console.WriteLine(v1 + v2);      // Vector2 { X = 4, Y = 6 }
Console.WriteLine(v1 == v2);     // False

// readonly record struct - all properties immutable
readonly record struct Color(byte R, byte G, byte B);

S10.6 Inheritance and Pattern Matching

abstract record Shape;
record Circle(double Radius) : Shape;
record Rectangle(double Width, double Height) : Shape;
record Triangle(double Base, double Height) : Shape;

static double Area(Shape s) => s switch
{
    Circle c     => Math.PI * c.Radius * c.Radius,
    Rectangle r  => r.Width * r.Height,
    Triangle t   => 0.5 * t.Base * t.Height,
    _            => throw new ArgumentException("unknown shape")
};

Console.WriteLine(Area(new Circle(5)));          // 78.54
Console.WriteLine(Area(new Rectangle(4, 6)));    // 24
Console.WriteLine(Area(new Triangle(3, 8)));     // 12

S10.7 Example - All Together

// Records - immutable order pipeline with with-expressions.

record Product(string Name, decimal Price);
record OrderLine(Product Item, int Qty)
{
    public decimal Total => Item.Price * Qty;
}
record Order(string Customer, IReadOnlyList<OrderLine> Lines)
{
    public decimal GrandTotal => Lines.Sum(l => l.Total);
}

var widget  = new Product("Widget", 9.99m);
var gadget  = new Product("Gadget", 49.95m);

var order = new Order("Alice", new[]
{
    new OrderLine(widget, 3),
    new OrderLine(gadget, 1),
});

Console.WriteLine($"Customer: {order.Customer}");
foreach (var line in order.Lines)
    Console.WriteLine($"  {line.Item.Name} x{line.Qty} = {line.Total:C}");
Console.WriteLine($"Total: {order.GrandTotal:C}");

// Update price without mutating the original
var discountedWidget = widget with { Price = 7.99m };
Console.WriteLine($"Sale price: {discountedWidget.Price:C}");

S10.8 Exercise

S10.9 Common Mistakes

record BadPoint(double X, double Y)
{
    public double Z { get; set; }   // mutable - excluded from equality!
}

var a = new BadPoint(1, 2) { Z = 3 };
var b = new BadPoint(1, 2) { Z = 9 };
Console.WriteLine(a == b);   // True - Z is not in equality check

S10.10 Key Terms