C# 9.0 Records: Immutable Data Types Done Right

Immutability is a cornerstone of robust distributed systems, but achieving it in C# has historically required copious boilerplate. C# 9.0 introduces Records, a first-class citizen for immutable data. Records provide value-based equality, non-destructive mutation, and concise syntax.

The Syntax Evolution

// Before (Verbose)
public class Person : IEquatable<Person>
{
    public string Name { get; }
    public int Age { get; }
    public Person(string name, int age) => (Name, Age) = (name, age);
    
    // + GetHashCode(), Equals(), == operator override...
}

// After (C# 9 Record)
public record Person(string Name, int Age);

Value-Based Equality

Records check equality by properties, not reference.

var p1 = new Person("Alice", 30);
var p2 = new Person("Alice", 30);

Console.WriteLine(p1 == p2); // True! (Unlike classes)

Non-Destructive Mutation (‘with’ expressions)

To modify an immutable object, you create a copy with specific changes.

var p3 = p1 with { Age = 31 };
// p1 remains unchanged. p3 is a new object.

Key Takeaways

• Use **Records** for DTOs, Messages, and Event payloads.
• Records compile to classes with synthesized equality members.
• Records support inheritance, unlike structs.