Why do plain values work in initializers but Camera.main does not?

Values like 5f or new List () do not touch the engine. Camera.main, Time.time, Random.Range and GetComponent ask the engine for something, and it is not ready while the object is being constructed.

Is Awake or Start the right place?

Use Awake for references the object needs itself, and Start for anything that depends on other objects having finished their own Awake.

Does this apply to static fields?

Yes. A static field initialized with a Unity API runs when the class is first loaded, which can be just as early. Initialize it from Awake or with [RuntimeInitializeOnLoadMethod].

UnityException: X is not allowed to be called from a MonoBehaviour constructor

ConsoleRuntime exception · Scripting

UnityException: X is not allowed to be called from a MonoBehaviour constructor (or instance field initializer), call it in Awake or Start instead.

Unity creates components while loading, sometimes off the main thread, before the engine is ready for API calls. Field initializers and constructors run at that moment, so most Unity APIs refuse to run there.

Read the Console line first

X in the message is the API that was called, such as get_main for Camera.main or get_time for Time.time, and the message ends with the script and GameObject that called it.

Common causes

A field initializer calls a Unity API

Initializers run as part of construction, so they hit the same rule.

 Example
C# 
 private Camera cam = Camera.main;          // not allowed
private float spawnTime = Time.time;       // not allowed
private int seed = Random.Range(0, 1000);  // not allowed
 3 lines

The class has a constructor

MonoBehaviours and ScriptableObjects should not use constructors for setup at all; Unity may call them more than once, including in the Editor.

The fix

 Fix
C# 
 private Camera cam;
private float spawnTime;

void Awake()
{
    cam = Camera.main;
    spawnTime = Time.time;
}
 8 lines

Stop it coming back

Initialize fields with plain values only (numbers, strings, new lists), and do anything that touches the engine in Awake, OnEnable or Start. For ScriptableObjects, use OnEnable.

Questions

Why do plain values work in initializers but Camera.main does not?: Values like 5f or new List<int>() do not touch the engine. Camera.main, Time.time, Random.Range and GetComponent ask the engine for something, and it is not ready while the object is being constructed.
Is Awake or Start the right place?: Use Awake for references the object needs itself, and Start for anything that depends on other objects having finished their own Awake.
Does this apply to static fields?: Yes. A static field initialized with a Unity API runs when the class is first loaded, which can be just as early. Initialize it from Awake or with [RuntimeInitializeOnLoadMethod].

Scripts that avoid it

Related errors

Got a different error? Paste it into the error lookup.