Unity in this post refers to the game engine, not the Microsoft framework, nor the DWM.

The first page of google for the query: “unity engine dependency injection” tickle me so much I have to write this up to rant about it.

Every single link on the 1st page, except the pdf one which I skipped, is either full of straight-up false information about DI, or kind of correct but completely miss the main point.

Common Misconceptions

Misconception #1: It too complex, I don’t need something to auto-connect dependencies for me, Reflection bad,…

The biggest misconception about DI stems from the fact that people are confusing DI for one of the DI Framework. You know the type: phone book-sized documentation, frequent name dropping (IoC, SOLID, …).

Now, these frameworks have their use, but as they’re usually the top results on Google, they also scare away many beginners from what is an extremely simple and useful design pattern.

> You don’t need any framework to use DI, and it’ll be a lot simpler without them.

Because of this confusion, many believe the purpose of DI is to avoid passing dependencies around. Instead, you conjure them by marking stuff with attributes, and they will be magically summoned by the dark power of Reflection.

> Dependency Injection has nothing to do with NOT passing things around. It’s the complete opposite.

> Dependency Injection has nothing to do with Reflection. It does not slow your code down. Unless you writing for an ATTiny where extra jumps to subroutine matter. Also, Reflection probably won’t slow your game down either, unless you use them every frame. It one of those strangely popular cargo-cult.

Misconception #2: DI is mostly used if you need unit testing

By using DI, you can swap out dependencies of a class without modifying it.

This is crucial when you want to do unit test, and is a big advantage of DI. However, being able to swap dependencies is a happy side-effect, but it not the main point.

If you don’t do unit testing, then first, you will never hear the end of it from people who do test, they believe it to be the second coming of Christ (not without reason). And second, you will still reap plenty of benefits from using DI.

Almost Misconception #3: Unity Editor is a dependency injector

This one is kind of true. Unity Editor is indeed a dependency injector. It allows you to inject stuff into other stuff by dragging and dropping. This is invaluable when you work with non-technical designers, and sometimes it actually is enough.

Other times, you want to have dependencies on things that are not Unity Object.

Or maybe you want things that are spawned at runtime to refer to things that also spawned at runtime.

Or maybe you simply forgot to drag the right thing in the right place. The bigger your project grows, the more likely that this will happen.

So what is Dependency Injection, and why do I want to use it?

Let say you want to let players select items in your games:

interface ISelector {
	void UpdatePosition(Vector2 newPosition);
	GameObject[] GetSelected();

ISelector selector;

void Start()
	selector = new SomeSelector();

void OnDrag(PointerEventData pointer)
	selected = selector.GetSelected();

You implement SomeSelector as a “normal” C# class - not a MonoBehaviour - because it just logic and have nothing to do with Unity. It get the list of items to select from ItemManager, which is a Singleton.

Error: NullReferenceException: player...

Right, ItemManager needs a reference to Players. As Players are spawned at runtime, you forgot about it - it not like there is an empty slot on the ItemManager inspector to remind you. Just create the PlayerSpawner then.

Error: NullReferenceException: inventoryPanel...

What? Ah, Players also need Inventory to store the items they picked up.You haven’t written the code to pick up items yet, so you think you can do it later. But no, ItemManager wants to do something with them in Awake.

The Inventory needs to reference the inventoryPanel to display what it contains. You have to remember to create one and put it in the right inspector slot.

Most of the time, these errors come from objects being created or methods being executed in the wrong order. They’re difficult to track down, as they only show up when the references are used, not when they’re created/assigned. It usually take a lot of time to track down these error, and it requires deep knowledge of the codebase.

You can remember the necessary details, like in the example above, if you just wrote Player and Inventory last week. But what about a year down the line? What about when someone else works on your project? Maybe a new team member, or a programmer from another studio that was hired to port your game to other platforms. They’ll either have to dig through thousands of lines of code, or waste time communicating back and forth.

Why does this happen?

The reason that these NullReferenceException happen is, most likely, because your variable are not properly initialized. Either you forgot to, or they’re initialized in the wrong order.

When you use global variables, this happens a lot. You can change a global from anywhere, at any time, so there is no guarantee that what you need will be where you expect it, when you need it.

Because of this, when you look at a class, you have no idea what are its dependencies, it can depend on every class for all you know. Use of global variable obscure all dependencies.

Singleton is the herald of global variables. When you use a Singleton, you’re not just using 1 global variable. You also make everything referenced by the Singleton, and anything referenced by those references and so on, global.

Dependency Injection to the rescue

Here how you can implement DI for the example above:

// SomeSelector.cs
public SomeSelector(List<Item> itemsToSelect){}

// Somewhere else
public GameObject inventoryPanel;

var inventory   = new Inventory(inventoryPanel, ...);
var players     = new []{new Players(inventory)};
var itemManager = new ItemManager(players, ...);
var selector    = new SomeSelector(itemManager.itemList);

There’re really just 2 step to implement DI:

  • Define what a class need, here using constructor arguments.
  • Pass in (inject) these things when an instance of the class is constructed.

As you can see, there is no way for you to create SomeSelector without having the list of items to select, which have to come from ItemManager. In turn, ItemManager cannot be created without the list of Players, and so on.

Not only that, there is no way for you to mess up the ordering. To create a Player, you need to provide it with an Inventory. There is no way to create the Inventory after the Player, so the Player can be sure that they always have an Inventory when they need it.

What Dependency Injection does, is forcing you to be explicit about your dependencies

Unfortunately, the code above probably can’t be used most of the time in Unity. You need MonoBehaviour in Unity, and you can’t use the constructor of a MonoBehaviour. This is what I usually have to do to work around it:

class Player : MonoBehaviour{
	void Init(Inventory inventory, ...) {};

player = Instantiate(playerPrefab);
player.Init(inventory, ...)

We’ll need 2 lines to create and initialize an object. We have to remember to always call Init, and can’t have the compiler yelling at us when we forget to pass in the dependencies. This is ugly, I have no better solution for this, and would love to hear if you have one. That said, I believe this to be the better long term solution than the invisible web that is Singleton.

Common Concern

What if my class has hundreds of dependencies? Do I have to have hundreds of arguments for my constructor/Init method?

You should never have hundreds of dependencies for a class. Instead you should split it up to multiple smaller classes, initialize those smaller class and pass the initialized instance into the big class. This is called Inversion of Control (IoC).

For example, your Player can walk, run, make sounds, attack, pick things up, … But it does not need the reference to everything that is needed for all of these activities.

Instead, you have separated class to handle each activity. For example, PlayerAnimation, which is initialized with AnimationClip and whatnot, then pass it to the Player. When PlayerAnimation needs to talk to others, say PlayerMovement, they can do so through Player.

A common design would be for Player to hold its states, such as walking or running. Some classes would write to these states, such as PlayerInput or PlayerMovementImpairingEffect. PlayerAnimation and PlayerMovement can both read from these states.

What if I need an object in many places, do I have to pass it everywhere?

Most of the time, yes. Remember that by not passing things around, you do not eliminate the need to do so, you only hide it.

When not to use Dependency Injection

The biggest disadvantage of DI is how verbose it is, and how much ritual you have to do before anything productive.

In this regard, I see a lot of similarity between the DI - Singleton fight and the Dynamic - Static typing one.

If you expect your dependency to be minimal, or if you’re in the exploratory phase of a project, Singleton is a great way to let you experiment with primary logic. What importance is that you’re aware of their shortcoming, and possible solutions.