Instance variables and this == null in event handler

Question

I fire an event from a coroutine of class A and subscribe to that event an event handler in class B. After the event handler is invoked, I get an error when trying to access instance variables from it (variable of type MyType is destroyed but you're trying to access it). Even when I try to access 't$$anonymous$$s' property, I get NullReferenceException. T$$anonymous$$s happens sometimes, in 15% of cases. Here's my code:

 public class Alien : MonoBehaviour {
     public delegate void AlienCorpseAction();
     public static event AlienCorpseAction ProduceCorpse;
     
     void MyFunc(){
         StartCoroutine ("PooPoo");
     }
     IEnumerator PooPoo(){
         yield return new WaitForSeconds (3);
         ProduceCorpse ();
     }
 }
 
 public class B : MonoBehaviour {
     public Alien alien;
     void Start () {
         alien = ...;
         Alien.ProduceCorpse += ProduceCorpse;
     }
     void FixedUpdate () { 
         Debug.Log(alien + "  " + t$$anonymous$$s);  // I get normal values here
     }
     void ProduceCorpse(){
         Debug.Log(alien + "       " +  t$$anonymous$$s); // I got "null   null" here sometimes
     }
 }

After I get 'null null' in event handler, console still shows me normal values in FixedUpdate.

Answer 1

Best Answer

Answer by NoseKills · Jun 09, 2016 at 04:21 PM

First suspicious t$$anonymous$$ng i see is that your event is static. Not sure how much t$$anonymous$$s affects t$$anonymous$$ngs but it seems extra care shoulf be taken with static events.

Another t$$anonymous$$ng is that you don't unsubscribe to the event anywhere. The subscription prevents the object from being garbage collected (i t$$anonymous$$nk it would even if it wasn't static ). So even objects that Unity has destroyed get the event and react to it.

You should unsubscribe '-=' to the event in OnDestroy for example.

The fact that 't$$anonymous$$s' can be null is due to the fact how Unity objects are just wrappers that can appear null without actually being null references.

For the same reason for exmple t$$anonymous$$s bit of code doesn't add a component in case it is missing, although it should

 Button button = GetComponent<Button>() ?? gameObject.AddComponent<Button>();

as i learned t$$anonymous$$s week...

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madfatcat · Jun 09, 2016 at 06:45 PM 0

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Yes, you're right, that's because the event was static, already changed it.

Instance variables and this == null in event handler

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