Unity C# Scripting (MonoBehaviour)

Write correct, idiomatic gameplay scripts in Unity 6. Get the lifecycle, component access, serialization, and coroutines right so behaviour is deterministic and the Inspector stays useful. Targets Unity 6.3 LTS (6000.3), C# / .NET Standard 2.1.

When to use

• Use when authoring or fixing a MonoBehaviour: choosing the right lifecycle callback, reading/caching components, exposing fields to the Inspector, or running timed logic with coroutines.
• Use when the project has *.cs files, an Assembly-CSharp or *.asmdef, and a ProjectSettings/ folder.

When not to use: moving rigidbodies / collision response → unity-physics; reading player input → unity-input-system; shared data assets / config → unity-scriptableobjects; Animator parameters → unity-animation. This skill owns the script lifecycle and C# plumbing, not those subsystems.

Core workflow

1. Pick the callback by purpose, not habit. Awake (cache references, runs once on load), OnEnable (subscribe to events), Start (init that depends on other objects' Awake), Update (per-frame logic/input polling), FixedUpdate (physics), LateUpdate (camera follow after movement), OnDisable/OnDestroy (unsubscribe/cleanup).
2. Cache component lookups in Awake — never call GetComponent every frame.
3. Expose tunables with [SerializeField] private, not public fields, so other code can't mutate them but designers can edit them in the Inspector.
4. Scale per-frame values by Time.deltaTime in Update (and Time.fixedDeltaTime semantics are automatic in FixedUpdate).
5. Use coroutines for time-sequenced logic (delays, tweens, "do X then wait then Y"); start them with StartCoroutine and stop them deterministically.
6. Verify in Play mode: check the Console for null-reference exceptions, confirm values in the Inspector update as expected, and watch the Profiler if Update is hot.
7. Check .meta files before you finish. When you create, move, rename or delete scripts or assets outside the Editor, always move a file together with its .meta (a new .meta means a new GUID and broken references). Then run python scripts/check_unity_project.py <project-dir> (--json for machine-readable output), fix every ERROR (duplicate or malformed GUIDs, files moved without their .meta, missing script references) and re-run until it exits 0. A missing .meta on a brand-new file is only a warning; Unity creates it on the next import. Run the script path relative to this skill directory, or resolve the installed skill path first. It needs Python 3.10+ and nothing else, reads only text-serialized assets, and does not replace opening the project in Unity.

Patterns

1. Lifecycle + cached components (the canonical skeleton)

using UnityEngine;

[RequireComponent(typeof(Rigidbody))]      // auto-adds the dependency, prevents null refs
public class PlayerController : MonoBehaviour
{
    [SerializeField] private float moveSpeed = 6f;   // editable in Inspector, private in code
    private Rigidbody _rb;                            // cached, not fetched per frame

    private void Awake() => _rb = GetComponent<Rigidbody>();  // cache once on load

    private void Update()
    {
        // Per-frame, non-physics work. Scale by deltaTime so it is frame-rate independent.
        transform.Rotate(0f, 90f * Time.deltaTime, 0f);
    }

    private void FixedUpdate()
    {
        // Physics work belongs here (fixed timestep). See the unity-physics skill.
        _rb.MovePosition(_rb.position + transform.forward * moveSpeed * Time.fixedDeltaTime);
    }
}

2. Safe component access with TryGetComponent

// Avoids allocating a null and is clearer than GetComponent + null check.
if (other.TryGetComponent<Health>(out var health))
    health.Apply(-10);

3. Serialization that shows up correctly in the Inspector

[SerializeField, Range(0f, 1f)] private float volume = 0.8f;  // slider
[SerializeField] private string playerName = "Hero";          // private but serialized

[System.Serializable]                 // REQUIRED for a plain class to serialize/show
public class Stats { public int hp = 100; public int mana = 50; }

[SerializeField] private Stats stats = new();  // nested struct-like data in the Inspector

4. Coroutines for time-sequenced logic

private void Start() => StartCoroutine(FlashThenHide());

private System.Collections.IEnumerator FlashThenHide()
{
    yield return new WaitForSeconds(0.5f);   // wait half a second of game time
    GetComponent<Renderer>().enabled = false;
    yield return null;                       // resume next frame
}

Pitfalls

• GetComponent in Update — it searches every frame and tanks performance. Cache the reference in Awake/Start.
• Physics in Update — moving a Rigidbody with forces or MovePosition outside FixedUpdate causes jitter and timestep-dependent behaviour. Read input in Update, apply physics in FixedUpdate.
• Relying on Start order across objects — Start runs after all Awakes, but order among Starts is undefined. Do cross-object wiring in Start, self-setup in Awake.
• public fields just to show them in the Inspector — that also lets any script mutate them. Use [SerializeField] private instead.
• gameObject.tag == "Enemy" allocates a string and is slower; use gameObject.CompareTag("Enemy").
• Coroutines stop when the GameObject is disabled — a disabled object's coroutines are killed; re-StartCoroutine in OnEnable if it must survive toggling. Setting only the script's enabled = false does not stop them.
• Obsolete warnings on FindObjectOfType / FindObjectsOfType — Unity 6 replaced them. Use FindFirstObjectByType (old behaviour), FindAnyObjectByType (faster, any match) or FindObjectsByType(FindObjectsSortMode.None); all are slow, so cache the result.
• An async method keeps running after its object is destroyed — await is not tied to the GameObject the way a coroutine is. Pass destroyCancellationToken to the Awaitable waits (they then throw OperationCanceledException), and never await the same Awaitable instance twice: instances are pooled.
• Update never runs before Start, but the first Update can run on the same frame as Start — guard against not-yet-initialised fields if you split setup oddly.

References

• For the full event-execution-order table, advanced coroutine patterns (custom CustomYieldInstruction, stopping by handle, WaitUntil/WaitWhile) and the Awaitable equivalents of coroutine waits, read references/lifecycle-and-coroutines.md.
• scripts/check_unity_project.py — read-only .meta/GUID and missing-script checker; --help lists every check and the exit codes.
• Primary docs: Unity Manual "Event function execution order" (https://docs.unity3d.com/Manual/execution-order.html), ScriptReference/MonoBehaviour, and the Manual page "Asynchronous programming with the Awaitable class".

Related skills

• unity-physics — Rigidbody, collisions, and FixedUpdate motion.
• unity-input-system — reading player input into these scripts.
• unity-scriptableobjects — sharing data/config between scripts without singletons.