Classes, structs, and records

C# has two families of types, and TS has one. A reference type (class) lives on the heap; a variable holds a reference to it, and two variables can point at the same object. A value type (struct) is copied by value; assigning it makes a fresh copy. Every object in JS/TS behaves like a reference type, so this distinction is new.

Then there's the record, which is the one you'll reach for most on the backend. A record is a reference type (usually) that the compiler gives value equality: two records are equal when their properties are equal, not when they're the same instance. That's the behaviour you probably assumed objects had coming from a world of deep-equal and immutable data. It's also concise: a positional record declares its shape in one line.

// reference type: identity equality, mutable by default
public class Flight { public string Number { get; set; } = ""; }

// value type: copied on assignment
public struct Coord { public double Lat; public double Lng; }

// record: reference type WITH value equality, made for data
public record EjError(string Field, string Message);

var a = new EjError("Value", "required");
var b = new EjError("Value", "required");
// a == b is TRUE for the record. For the class it would be false.

EjError above is real Fusion domain code. It's a record because two errors with the same field and message are the same error; you never care which instance you're holding. That's the tell for reaching for a record: it's data, defined by its values, and you want equality and immutability to come for free.