You are trying to create a MonoBehaviour using the 'new' keyword. This is not allowed

ConsoleRuntime exception · Components

You are trying to create a MonoBehaviour using the 'new' keyword. This is not allowed. MonoBehaviours can only be added using AddComponent().

A MonoBehaviour only works when it is attached to a GameObject, because Unity creates, updates and destroys it as part of that object. new gives you a C# object with no GameObject, so Unity warns and the object never gets Update, Start or a transform.

Read the Console line first

The stack trace names the line with new YourScript().

Common causes

  1. Creating a component with new

    To give an object a new component at runtime, call AddComponent on its GameObject.

  2. Using a MonoBehaviour as a data holder

    If the class only stores data and has no Update or transform, it does not need to be a MonoBehaviour. Make it a plain [System.Serializable] class or a ScriptableObject.

The fix

Fix
C# 
// A component, attached to an object
var health = enemy.AddComponent<Health>();

// A data holder, created with new
[System.Serializable]
public class ItemStack { public string id; public int count; }
var stack = new ItemStack { id = "gem", count = 3 };
7 lines

Stop it coming back

Decide early whether a class is a component (it lives on a GameObject) or data (it does not), and only derive from MonoBehaviour for the first.

Questions

Why does the object I made with new seem to work at first?
You get a C# object with no native side: it has no GameObject, its transform is null, and Unity never calls its Awake, Start or Update. Anything that touches the engine fails later. Add it with gameObject.AddComponent<T>() instead.
I only need a class to hold data. What should I use?
A plain C# class without : MonoBehaviour. Mark it [System.Serializable] and it shows up in the Inspector as a field of the component that holds it, and JsonUtility can save it.
Is the same true for ScriptableObject?
Yes. Create ScriptableObjects with ScriptableObject.CreateInstance<T>(), or as assets through a [CreateAssetMenu] menu item. Unity warns if you use new.

Scripts that avoid it

Related errors

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