Home › C# Concepts › Null & Nullable Types

Null

Keywords	`null`, `?.`, `??`, `??=`, `int?`, `is null`
Category	Type system
In Unity	Empty Inspector fields, destroyed objects, `GetComponent` finding nothing

Null & Nullable Types

null means "this variable doesn't point to any object". Using a null variable causes the most common error in Unity, NullReferenceException. This page covers how to check for null safely, the ?. and ?? shortcuts, and the important Unity exception to those shortcuts.

A reference variable is a leash. Normally there's a dog on the end. null is an empty leash. Calling a method on it (dog.Bark()) is telling the empty leash to bark, and C# throws NullReferenceException because there's no dog to do it.

Checking for null

[SerializeField] private Transform target;

void Update()
{
    if (target == null) return; // nothing to follow yet
    transform.LookAt(target);
}

A good habit: check required references once in Awake and log a clear message, instead of letting a mystery error happen later.

void Awake()
{
    if (target == null)
        Debug.LogError($"{name}: 'target' is not assigned in the Inspector.", this);
}

Null shortcuts (for normal C# objects)

Operator	Meaning	Example
`a?.B()`	Call only if `a` isn't null; otherwise result is null	`OnDied?.Invoke();`
`a ?? b`	Use `a`, or `b` if `a` is null	`string n = playerName ?? "Guest";`
`a ??= b`	Assign `b` only if `a` is null	`cache ??= new List<int>();`
`a is null` / `a is not null`	Pure null check	`if (data is null) return;`

The Unity exception: don't use these on Unity objects

?. ?? ??= and "is null" skip Unity's destroyed checkUnity objects (GameObject, components, assets) that have been destroyed still exist as C# objects. Unity overrides == so destroyedObj == null returns true. The operators ?., ??, ??= and is null don't use that override, so they think a destroyed object is still alive.

Destroy(enemy.gameObject);
// ...a frame later:

enemy?.TakeDamage(5);         // BUG: calls TakeDamage on a destroyed object -> MissingReferenceException
if (enemy != null) enemy.TakeDamage(5); // CORRECT

var cam = customCam ?? Camera.main;                   // BUG if customCam was destroyed
var cam2 = customCam != null ? customCam : Camera.main; // CORRECT

if (enemy) { }                // also correct: Unity objects convert to bool (false if null/destroyed)

Rule of thumb: Unity object → == null / != null. Plain C# object, event or delegate → any of the shortcuts. See also "fake null" on the Object page.

Nullable value types (int?, float?, Vector3?)

Value types like int and Vector3 can't normally be null. Add ? to allow it. Useful when "no value" is different from zero.

private Vector3? lastSeenPosition; // null = never seen the player

void Update()
{
    if (CanSeePlayer())
        lastSeenPosition = player.position;

    if (lastSeenPosition.HasValue)
    {
        MoveTowards(lastSeenPosition.Value);
    }
}

// Shorter:
float? bestTime = LoadBestTime();       // null = no record yet
float shown = bestTime ?? 0f;           // ?? is fine: float? is not a Unity object

Member	Description
`x.HasValue`	`true` if it holds a value
`x.Value`	The value. Throws `InvalidOperationException` if there is none.
`x.GetValueOrDefault()`	The value, or `0`/`default` if none

Unity can't show nullable fields (int?) in the Inspector.

What happens at runtime

A reference variable stores an address. null is address "nothing".
Calling a member on it makes the runtime detect the invalid access and throw NullReferenceException.
For Unity objects, the C# object can outlive the native engine object. After Destroy, the native part is gone; Unity's == checks for that. Accessing members then throws MissingReferenceException.
An unassigned serialized field in the Editor may also be a "fake null" and throw UnassignedReferenceException, whose message names the field. That's why the Editor's errors can be more helpful than a build's.

Common Mistakes

Assuming GetComponent always succeeds. It returns null if the component isn't there. Use TryGetComponent or [RequireComponent].
Forgetting to assign an Inspector field. The #1 cause of NullReferenceException for beginners.
Using a reference in Awake that another script sets in its Awake. Order between objects isn't guaranteed; use Start for cross-object references.
?. on Unity objects. See the warning above.