Immutability

Mutable shared state is the root cause of most concurrency bugs. Immutable values MUST be the default; introduce mutability only where necessary. Value types SHOULD be preferred over reference types. Mutation MUST be contained behind clear boundaries.

Kotlin

Use val by default. Use data class for value types. Introduce var only when mutation is required, and contain mutable state behind StateFlow.

TypeScript

Use const by default. Use var/let only when mutation is required. Prefer useState (React) for contained mutable state.

C#

Use readonly fields and readonly struct by default. Introduce mutability only when required.

• Prefer System.Collections.Immutable (ImmutableList<T>, ImmutableDictionary<K,V>) for shared collections
• Use record for DTOs, API responses, domain events, value objects
• Use record struct / readonly record struct for small immutable value types
• Prefer positional records (record Person(string Name, int Age)) for simple data carriers
• Use init setters and required keyword for mandatory properties
• Use with expressions for non-destructive mutation
• Contain mutable state behind ObservableObject (UI) or thread-safe wrappers
• Reserve class with mutable state for entities with identity semantics

// Immutable DTO
public record OrderSummary(string OrderId, decimal Total, DateTime CreatedAt);

// Modified copy
var updated = order with { Total = 99.99m };

// Readonly value type
public readonly record struct Point(double X, double Y);

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