Records, structs, and classes — when to use which in C# 10+_

My rule of thumb: class for mutable domain objects with identity, record for immutable data transfer objects where equality is by value, struct for small value types that are frequently stack-allocated. record struct exists too for value-type records.

The killer feature of records is non-destructive mutation via with. Instead of mutating an object you produce a copy with changed fields. This makes immutable pipelines trivial and is perfect for functional-style C# code.

Structs are copied on assignment. Passing a large struct to a method copies every field. This is the opposite of the performance optimization people expect. Keep structs small — a few primitive fields — or use ref parameters and Span.

C# 12 primary constructors mean you can write: public record UserDto(int Id, string Name, string Email); and get a fully immutable DTO with value equality, ToString, and deconstruction for free. This is genuinely one of the nicest things in modern C#.