Found a weird problem about AudioSource.PlayOneShot

Question

AudioSource _AudioSource_1 = new GameObject().GetComponent<AudioSource>();
AudioSource _AudioSource_2 = null;
Debug.Log(_AudioSource_1.Equals(_AudioSource_2));
_AudioSource_1.PlayOneShot(new AudioClip());
Debug.Log("Pass1");
_AudioSource_2.PlayOneShot(new AudioClip());
Debug.Log("Pass2");

Here is my code and what console displayed.
I'd appreciate it if someone could tell me why this
_AudioSource_1.PlayOneShot(new AudioClip()); can pass.

Answer 1

Answer by hexagonius · Feb 23, 2017 at 07:50 PM

I know that unity overloaded null, maybe equals also, for several reasons. you are, somehow, successfully using the constructor on the returned AudioSource. maybe it creates a new instance but overrides null so that it's not pointing to any legal C++ native component anymore effectively comparing nulls.
don't use new on any unity class. use AddComponent if you need another.

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Hlero · Feb 24, 2017 at 07:16 AM 0

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Thanks a lot! That makes sense to me. By the way, AudioClip is derived from Object, it can't be added to a GameObject.

Answer 2

Answer by Hlero · Feb 24, 2017 at 08:29 AM

I did some research and found, truly like @hexagonius said, unity overrided something like Equals, == and != to build a Null Object pattern.
So here is my code to test:

AudioSource _AudioSource_1 = new GameObject().GetComponent<AudioSource>();
Debug.Log(object.Equals(_AudioSource_1,  
FormatterServices.GetUninitializedObject(typeof(AudioSource))));
Debug.Log(_AudioSource_1.GetType());
Debug.Log(_AudioSource_1);
_AudioSource_1.PlayOneShot(null);
Debug.Log("Pass1");

And Console:

It looks like GetComponent<T> will always return reference of a T type instance if T is a NATIVE component type, whether or not there is a T component attached to the gameobject.

But when you want to get a custom component, if there is no one attached, GetComponent will definitely return a real null. Like this:

GameDataState _GameDataState = new GameObject().GetComponent<GameDataState>();
Debug.Log(object.Equals(_GameDataState, null));
Debug.Log(_GameDataState.enabled);
Debug.Log("Pass");

It would yield a NullReferenceException.
So you can see unity overrided some method to make you think it's null when you try to get a not attached component, and i think maybe they forgot to deal with this PlayOneShot method so it can be invoked.

nulltest.png (13.9 kB)

customecomponent.png (4.8 kB)

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hexagonius · Feb 24, 2017 at 10:09 AM 0

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it's just plain wrong to use new here. A GameObject must not be created through it's constructor. and now that I read more careful it's just that you call GetComponent on a new gameobject that simply returns null because there's no such component. So it's not even some strange overload, it's simply what GetComponent does when there's no component.

Hlero hexagonius · Feb 24, 2017 at 10:44 AM 0

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I use new just for test and i have tried to create a gameobject through inspector and find it, then i called GetComponent<AudioSource>().PlayOneShot(null), the result was same. Look the question my friend! My problem is when i call GetComponent<AudioSource> on a gameobject without AudioSource, it returns me a reference of an AudioSource instance, and i can call PlayOneShot method of it without any exception. It is supposed to be a NullReferenceException but it's not! That's what confused me ok?

Found a weird problem about AudioSource.PlayOneShot

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