Fields vs Properties

**Fields** are raw data; **properties** wrap them with `get`/`set` logic:

```csharp

class Person {

// auto-property: compiler generates the backing field

public string Name { get; set; } = "";

public int Age { get; set; }

// computed property — no backing field

public bool IsAdult => Age >= 18;

}

var p = new Person { Name = "Alice", Age = 25 };

Console.WriteLine(p.IsAdult); // true

```

---

init-Only Properties (C# 9+)

Set once during object initialisation:

```csharp

class Point {

public int X { get; init; }

public int Y { get; init; }

}

var pt = new Point { X = 3, Y = 4 };

// pt.X = 5; ← compile error after construction

```

---

Required Properties (C# 11+)

```csharp

class User {

public required string Email { get; set; }

}

// var u = new User(); ← compile error if Email not set

var u = new User { Email = "a@b.com" };

```

---

Backing Field with Validation

```csharp

class BankAccount {

private decimal _balance;

public decimal Balance {

get => _balance;

set {

if (value < 0) throw new ArgumentOutOfRangeException(nameof(value));

_balance = value;

}

}

}

```

---

Value vs Reference Semantics

**struct** = value type (copied on assignment):

```csharp

struct Vector2 { public float X, Y; }

Vector2 a = new Vector2 { X = 1, Y = 2 };

Vector2 b = a; // copy

b.X = 99;

Console.WriteLine(a.X); // still 1

```

**class** = reference type (shared on assignment):

```csharp

class Box { public int Value; }

Box a = new Box { Value = 1 };

Box b = a; // same object

b.Value = 99;

Console.WriteLine(a.Value); // 99!

```

---

records — Immutable Value Objects (C# 9+)

```csharp

record Product(string Name, decimal Price);

var p1 = new Product("Book", 9.99m);

var p2 = p1 with { Price = 12.99m }; // non-destructive mutation

Console.WriteLine(p1.Price); // 9.99

Console.WriteLine(p2.Price); // 12.99

```

---

What's Next?

You understand C# data modeling primitives. Next: **structs and classes** — constructors, inheritance, encapsulation.