Component and execution attributes

Unity 6 reference

At a glance

Class-level attributes express component dependencies, edit-time behavior and execution preferences. They improve authoring workflows, but should not replace explicit runtime checks.

Quick reference

Attribute	What it does	Applies to and requires
[AddComponentMenu("…")]	Choose the path where a component appears in Add Component.	Class · UnityEngine
[DisallowMultipleComponent]	Prevent more than one compatible instance on the same GameObject.	Class · UnityEngine
[RequireComponent(typeof(X))]	Ensure a required component is added when attaching this script.	Class · UnityEngine
[ExecuteInEditMode]	Run a MonoBehaviour in Edit mode using the older mechanism.	Class · UnityEngine · Legacy
[ExecuteAlways]	Allow MonoBehaviour callbacks in Edit mode and Prefab Mode.	Class · UnityEngine
[SelectionBase]	Choose the parent object when clicking a child in Scene view.	Class · UnityEngine
[HelpURL("url")]	Attach a documentation link to the component help icon.	Class · UnityEngine
[DefaultExecutionOrder(n)]	Set relative default execution order for a MonoBehaviour class.	Class · UnityEngine
[AssemblyIsEditorAssembly]	Mark an entire assembly as Editor-only.	Assembly · UnityEngine
[MovedFrom(...)]	Help Unity remap serialized references after moving a type or namespace.	Type · UnityEngine.Scripting.APIUpdating

How to choose

[AddComponentMenu("…")]

Choose the path where a component appears in Add Component.

Use it when: Group your custom components under an easy-to-find gameplay menu.

Be aware: It changes discoverability, not component behavior.

Reference: Unity documentation

[DisallowMultipleComponent]

Prevent more than one compatible instance on the same GameObject.

Use it when: Mark a scene controller that should exist only once per GameObject.

Be aware: It works at component attachment time; verify inheritance behavior for base components.

Reference: Unity documentation

[RequireComponent(typeof(X))]

Ensure a required component is added when attaching this script.

Use it when: Ensure your controller has the Collider or Rigidbody it depends upon.

Be aware: Does not continuously repair an already-existing component missing a dependency.

Reference: Unity documentation

[ExecuteInEditMode]

Run a MonoBehaviour in Edit mode using the older mechanism.

Use it when: Understand or maintain legacy editor scripts while planning migration.

Be aware: Unity recommends ExecuteAlways for Prefab Mode-compatible workflows.

Reference: Unity documentation

[ExecuteAlways]

Allow MonoBehaviour callbacks in Edit mode and Prefab Mode.

Use it when: Update safe editor previews for objects and prefabs without entering Play mode.

Be aware: Callbacks behave differently outside Play mode. Guard changes to scenes and prefabs carefully.

Reference: Unity documentation

[SelectionBase]

Choose the parent object when clicking a child in Scene view.

Use it when: Make a character or machine easier to select as a whole in Scene view.

Be aware: This affects editor selection, not runtime input or hierarchy relationships.

Reference: Unity documentation

[HelpURL("url")]

Attach a documentation link to the component help icon.

Use it when: Link a complicated component to its maintenance guide.

Be aware: Link to a stable HTTPS page documenting this component.

Reference: Unity documentation

[DefaultExecutionOrder(n)]

Set relative default execution order for a MonoBehaviour class.

Use it when: Set a documented order between two lifecycle systems when unavoidable.

Be aware: Prefer explicit dependencies; script execution order should not become a hidden coupling mechanism.

Reference: Unity documentation

[AssemblyIsEditorAssembly]

Mark an entire assembly as Editor-only.

Use it when: Maintain a manually configured Editor assembly; prefer asmdefs for new projects.

Be aware: In typical Unity projects, an Editor-only asmdef is usually easier to maintain.

Reference: Unity documentation

[MovedFrom(...)]

Help Unity remap serialized references after moving a type or namespace.

Use it when: Move a serializable class or namespace while helping Unity migrate references.

Be aware: Test migration with real serialized assets and source metadata.

Reference: Unity documentation

Expressing dependencies

Unity C# | PickupSensor.cs

using UnityEngine;

[DisallowMultipleComponent]
[RequireComponent(typeof(SphereCollider))]
[AddComponentMenu("Gameplay/Pickup Sensor")]
public sealed class PickupSensor : MonoBehaviour
{
    private SphereCollider trigger;

    private void Awake()
    {
        trigger = GetComponent<SphereCollider>();
        if (trigger == null)
        {
            Debug.LogError("PickupSensor requires a SphereCollider.", this);
            enabled = false;
            return;
        }
        trigger.isTrigger = true;
    }
}

RequireComponent helps when the script is first attached, and the explicit check explains the failure if a scene or prefab later becomes inconsistent. Use ExecuteAlways only if edit-time execution is actually required.

Prefab safety

ExecuteAlways does not mean disabled components run normally. Guard code that could modify prefab assets outside Play mode.

Related lessons

• Unity scripting lifecycle
• Assembly definitions
• Namespaces