Records, Structs and Equality

Data-centric types

A record is a type designed for data. public record Customer(string Id, string Name); is a positional record: the compiler generates init-only properties, a constructor, deconstruction, a readable ToString() and, most importantly, value-based equality: two records are equal if all their properties are equal, whereas two ordinary class instances are equal only if they are the same object. Records support non-destructive mutation with with: var renamed = customer with { Name = "Ravi" }; creates a copy with one property changed. record (or record class) is a reference type; record struct is a value type with the same conveniences. Records suit DTOs, messages, value objects and events. A record holding a List<T> is still only shallowly immutable, and its equality compares the list reference, not its contents.

Value equality and with-expressions

Equality compares contents, not identity.

public record Money(decimal Amount, string Currency)
{
    public Money Add(Money other) =>
        other.Currency == Currency
            ? this with { Amount = Amount + other.Amount }
            : throw new InvalidOperationException("Currency mismatch");
}

var a = new Money(100m, "INR");
var b = new Money(100m, "INR");
Console.WriteLine(a == b);              // True: same values
Console.WriteLine(ReferenceEquals(a, b)); // False: different objects

var (amount, currency) = a.Add(b);    // deconstruction
Console.WriteLine($"{amount} {currency}");

Records in dictionaries and sets

Because equality and GetHashCode are based on values, records work well as dictionary keys, but only if they are truly immutable. Mutating a key after inserting it makes the entry impossible to find.

त्वरित जाँच: Two record instances are created with identical property values. What does == return?

• False, because they are different objects
• It does not compile
• True, because records use value-based equality
• It depends on the garbage collector