Unity Project Architecture

Overview

Unity makes it easy to put everything in one MonoBehaviour that finds its collaborators with FindObjectOfType and a static Instance. That works for a jam and collapses after a few months: nothing is testable, compile times grow, and scene order decides behaviour. Structure the project as assemblies with a dependency direction, plain C# for rules, thin components for engine glue, and data in assets.

Core principle: Rules in plain C# assemblies, components as adapters, data in ScriptableObjects, dependencies passed in. The repository's existing layout and DI choice win.

Assemblies and direction

Game.Core      (noEngineReferences: rules, math, state machines, save DTOs)
   ↑
Game.Runtime   (MonoBehaviours, ScriptableObjects, adapters; references Core)
   ↑
Game.UI        (views and presenters; references Runtime/Core, never the reverse)
Game.Editor    (includePlatforms: Editor; tools, validators)
Tests.EditMode / Tests.PlayMode (test assemblies; see unity-testing)

{
  "name": "Game.Core",
  "rootNamespace": "Game.Core",
  "references": [],
  "noEngineReferences": true,
  "autoReferenced": false
}

• An assembly per layer keeps compile times down and makes a wrong-way reference a compile error instead of a review comment.
• Game.Core with noEngineReferences: true can be compiled and tested by plain dotnet test when the editor is missing (testable-game-logic).
• Gameplay never references UI; UI observes gameplay through events.

Components

// ❌ lookups every frame, public mutable fields, rules inside the component
public class Enemy : MonoBehaviour {
    public int hp = 100;
    void Update() {
        var player = GameObject.Find("Player");
        if (Vector3.Distance(transform.position, player.transform.position) < 5) GetComponent<Animator>().SetTrigger("Attack");
    }
}

// ✅ cached references, serialized private fields, rule delegated to plain C#
[RequireComponent(typeof(Animator))]
public sealed class EnemyView : MonoBehaviour {
    static readonly int AttackHash = Animator.StringToHash("Attack");
    [SerializeField] EnemyDefinition definition;
    Animator _animator; EnemyBrain _brain; ITargetProvider _targets;

    public void Construct(ITargetProvider targets) => _targets = targets;
    void Awake() { _animator = GetComponent<Animator>(); _brain = new EnemyBrain(definition.AttackRange); }
    void Update() {
        if (_brain.ShouldAttack(transform.position, _targets.PlayerPosition)) _animator.SetTrigger(AttackHash);
    }
}

• [SerializeField] private over public fields; public API through properties and methods.
• Awake initialises the object itself; OnEnable/Start wire up to others; OnDisable unsubscribes everything OnEnable subscribed.
• [RequireComponent] for hard dependencies on the same GameObject; TryGetComponent instead of GetComponent + null check.
• Physics in FixedUpdate through the Rigidbody API (MovePosition, AddForce, linearVelocity), never transform.position on a dynamic body.

Data and events in ScriptableObjects

• Definitions (weapons, enemies, levels) are ScriptableObjects (data-driven-gameplay-design); runtime state is a separate object built from them. Changes made to a ScriptableObject in Play mode persist in the editor — never write gameplay state into one.
• Event channels (GameEvent : ScriptableObject with Raise() and listeners) decouple scenes, but keep them few and named for what happened (PlayerDied), not what should happen.

Dependencies, not singletons

• Use the repository's container if it has one (VContainer, Zenject/Extenject): register services in a LifetimeScope/installer and inject via constructor (plain classes) or [Inject] method (components).
• Without one: a bootstrap scene or composition root creates services and passes them in (Construct(...) as above).
• No new static Instance for gameplay state. Engine-wide stateless helpers are fine as static classes.

Input, assets, async

• Input System, not the legacy Input class — actions from the .inputactions asset, rebinding and device glyphs (game-input-and-accessibility). A project still on the legacy Input Manager stays on it unless the task migrates it.
• Addressables for content loaded at runtime (game-asset-pipeline); no new content in Resources/. Every LoadAssetAsync handle is released.
• Async: Awaitable (Unity 6) or UniTask where the repo uses it; cancel with destroyCancellationToken so a destroyed object's continuation does not run. async void only for event handlers, with try/catch.

Serialization safety

• Renaming a serialized field loses its data in every scene and prefab unless you add [FormerlySerializedAs("oldName")].
• Polymorphic serialized fields need [SerializeReference]; plain [SerializeField] slices to the base type.
• Keep Asset Serialization on Force Text so scenes and prefabs diff and merge (UnityYAMLMerge); prefer prefabs and nested prefabs over large scene edits so changes stay reviewable.

Unity 6 API notes (follow the pinned version)

• Rigidbody.velocity → linearVelocity; drag/angularDrag → linearDamping/angularDamping.
• FindObjectsOfType/FindObjectOfType → FindObjectsByType(FindObjectsSortMode.None) / FindAnyObjectByType (and avoid both in gameplay paths).
• Object.InstantiateAsync for large prefabs; Awaitable.NextFrameAsync, Awaitable.WaitForSecondsAsync.
• Unity 6 LTS lines: 6000.0 LTS support ends in October 2026, 6000.3 LTS is supported to the end of 2027 — never upgrade the project on your own.

Common Mistakes

• GameObject.Find, FindAnyObjectByType or GetComponent in Update.
• A MonoBehaviour holding the rule logic, making it testable only in PlayMode.
• static Instance managers that depend on scene load order.
• Renaming serialized fields without FormerlySerializedAs.
• Subscribing in OnEnable without unsubscribing in OnDisable — leaks and calls on destroyed objects.

Red Flags

• A UI assembly referenced by gameplay code.
• A new Resources.Load call.
• transform.position = on an object with a non-kinematic Rigidbody.
• A component longer than a few hundred lines with rules, input and presentation mixed.