Why does C# null-conditional operator not work with Unity serializable variables?

问题

I've noticed that if I have some variables exposed to the Unity inspector such as:

[SerializeField] GameObject _tickIcon;

If I leave them unassigned and try to use the null conditional operator and call a method on that object I get an exception saying the variable is not assigned. So basically instead of doing this:

_tickIcon?.SetActive(false);

It's forcing me to do this:

if(_tickIcon != null)
{
   _tickIcon.SetActive(false)
}

So I'm guessing this must be something specific to unity's runtime, it's not really null, but I can check for null and it work. I don't really understand this.

回答1:

It does not work in general with anything inheriting from UnityEngine.Object!

It is bypassed due to how they internally implemented the ==/!= method differently. See Custom == operator, should we keep it?

ReSharper explained it pretty well in Possible unintended bypass of lifetime check of underlying Unity engine object

Object is in some occasions not really null but still keeps some meta data.

E.g. after

Destroy(_tickIcon);
_tickIcon.SetActive(false);

you will note that you don't get a normal NullReferenceException which would be the case if it were actually null but rather get a Unity customs MissingReferenceException telling you a probable reason for why the exception was thrown.

---

As solution UnityEngine.Object has the implicit bool operator

Does the object exist?

The example given there is actually not really true - see above

You should always check the existence of anything derived from UnityEngine.Object like this:

if(_tickIcon)
{
    _tickIcon.SetActive(false);
}

来源：https://stackoverflow.com/questions/62678228/why-does-c-sharp-null-conditional-operator-not-work-with-unity-serializable-vari