Unity: the null-coalescing operators not working

May 20, 2023

Problem

I cannot use ?? and ??= operators in Unity.

public class Compo : MonoBehaviour
{
    [SerializeField] Transform? _tr;

    void Start()
    {
        _tr ??= transform;
        Debug.Log(_tr);
    }
}

I attached this class to an object.

I did not attach any Transform to _tr property but the object’s transform should be assigned to _tr.

Eventually, I got null in the console.

Attempt

I tried not to use the operators like below.

if (_tr == null)
{
    _tr = transform;
}

Successfully, transform was assigned to _tr.

Then, I tried simpler code like below.

int? x = null;
x ??= 10;

I got 10.

Environment

OS: macOS 13.3.1 (a)（22E772610a）
Unity: 2021.3.22f1
Mono: 6.12.0.182
C#: .NET Standard 2.1

Additional Compiler Arguments

-nullable:enable

>Solution :

The null-coalescing and null-conditional (?. and ?[]) operators compare the reference of the left operand to null.

_tr ??= transform;

is equivalent to

if(object.ReferenceEquals(_tr, null))
    _tr = transform;

But all the Unity objects override the == operator so that even the references are not null, the objects can be equal to null. For example:

var r = GetComponent<Renderer>(); // suppose there is a renderer component attached
Destroy(r);
print(r == null); // true
print(object.ReferenceEquals(r, null)); // false

That’s why you should avoid to use these on Unity objects.