Question

I decided to implement a simple messaging system into my project. I’m using this one: CSharpMessenger Extended (it’s implemented with static methods).

It’s very strange that when I call a method directly everything is working correctly. But when I broadcast a message with the messaging system I get a NullReferenceException on some of the game objects. To my surprise adding the line if (_gameObject == null) return; solves the problem. However it’s not an option to add a check if the object is null to every place where I get this exception.

What might be the problem?

Here’s my code for broadcasting a message:

public class Head : MonoBehaviour {

    public Snake snake;

    void OnControllerColliderHit (ControllerColliderHit hit)
    {
        if ( hit.gameObject.GetComponent<Terrain>() )
            return;

            //This way everything was working without any surprises.
            //snake.PropCollected( hit.gameObject.GetComponent<Prop>() );
            //Using messaging system instead
        if ( hit.gameObject.GetComponent<Prop>() )
            Messenger<Prop>.Broadcast( "prop collected", hit.gameObject.GetComponent<Prop>() );
            Destroy(hit.gameObject);

        }
    }

Here’s how I subscribe to the event and respond to it:

public class Snake : MonoBehaviour {

    public GameObject headBlock;

    public GameObject snakeBlocks;

    int lastSnakeBlockId;

    private GameObject FindBlockWithId( int _id )
    {
            if (!snakeBlocks) return null;    //This line somehow solves the problem; However the object is 100% assigned in the editor.

            foreach (Transform child in snakeBlocks.transform) {
                if (child.gameObject.GetComponent<SnakeBlockScript>().blockId == _id)
                {
                    return child.gameObject;
                }
            }

        return headBlock;
    }

    void Awake()
    {
        //Set up message listeners
        Messenger<Prop>.AddListener("prop collected", AddBlock);    
    }

    public void AddBlock(Prop _prop)
    {
        GameObject lastBlock = FindBlockWithId(lastSnakeBlockId - 1);
        if (!lastBlock) return;

        //Some more code
        //...
    }
}

Thank you!

Answer 1

It sounds like you have a condition where “snakeBlocks” is re-initialized somewhere else in the application, and this thread happens to hit occasionally during that reference being set to something else.

Something Like:

snakeBlocks = null;

//Do some stuff.

snakeBlocks = someNewInstanceOfGameObject;

Just a thought.