Help with c# ?? operator problem.

Unity Engine
1

Hi,

i have:

myAudioSource = audio == null ? gameObject.AddComponent<AudioSource>() : audio; // working
// alternative
myAudioSource = audio ?? gameObject.AddComponent<AudioSource>();  // <--- not working?

NOTE: “audio” is a field from a baseclass

The first version was the org. code i copy&pasted, now VS + ReShaper “correctly” tells me that i can convert this expression to a ?? expression. The problem is that using the second expression the audiosource is not created?

I’m missing something, since from my point of view the two should be equal expressions?

Thx Andy

2

?? indicates that if the expression on left side is null, it will take the expression on the right side. It should work…

3

Unity created some equality/inequality checks as some implicit casts which may disturb this a little bit. null may in Unitys case (everything inherited from UnityEngine.Object) not be a real null.

4

If you’re right, why the first line is working? It’s also a null test.

5

The first one will call the equality operator from UnityEngine.Object, the second does a real null check. In Unity == null means that the object is either really null or marked as destroyed.

This:

GameObject a = null;
GameObject b = a == null ? new GameObject() : a;

Becomes:

IL_0001:  ldnull
IL_0002:  stloc.0
IL_0003:  ldloc.0
IL_0004:  ldnull
IL_0005:  call       bool [UnityEngine]UnityEngine.Object::op_Equality(class [UnityEngine]UnityEngine.Object,
                                                                     class [UnityEngine]UnityEngine.Object)
IL_000a:  brtrue.s   IL_000f
IL_000c:  ldloc.0
IL_000d:  br.s       IL_0014
IL_000f:  newobj     instance void [UnityEngine]UnityEngine.GameObject::.ctor()
IL_0014:  stloc.1

and this:

GameObject a = null;
GameObject b = a ?? new GameObject();

Becomes:

IL_0001:  ldnull
IL_0002:  stloc.0
IL_0003:  ldloc.0
IL_0004:  dup
IL_0005:  brtrue.s   IL_000d
IL_0007:  pop
IL_0008:  newobj     instance void [UnityEngine]UnityEngine.GameObject::.ctor()
IL_000d:  stloc.1

Edit:
Why this fails for the op is unclear to me, maybe audio has a stub value but is internal marked as not existing/destroyed.

6

Thanks for the informations.
Maybe it’s not working because audio is a shortcut to the component.
If you try GetComponent() it could be different…

7

audio does this internally :wink:

8

Thx for the infos,

“audio” is actually a default property from UnityEngine.Component

public AudioSource audio { [WrapperlessIcall, MethodImpl(MethodImplOptions.InternalCall)] get; }

I still don’t get why “??” fails, in this case.

9

As I never seen the code behind it, I can’t tell that, but I can trust you for sure.

10

audio might do that internally but it might not be the same pattern as transform because a GameObject must have a Transform whereas it can optionally have an AudioSource.