Unity3d c# NullReferenceException keeps showing up

Question

using UnityEngine;
using System.Collections;

public class SphereObj : MonoBehaviour {

    private GameObject manager;


    void Awake () {
        //things
    }

    public void SetParent(GameObject myManager) {
        manager = myManager;
    }
}

test

using UnityEngine;
using System.Collections;


public class SphereManager : MonoBehaviour {

    int position;
    int detail;

    public GameObject preSphere;

    void Start () {

        for(int i = 0; i < detail; i++){
            SphereObj itsMe = Instantiate(preSphere, new Vector3(0, 0, 0), Quaternion.identity) as SphereObj;
            itsMe.SetParent(gameObject);

        }
    }
}

So im building a game with basicly an unlimited amout of spheres and im instantiating all from a prefab. the prefabs got the first script attached managing sphere stuff, but i want them to have access to variables from the manager script, which is linked to an empty gameobject spawning things. my plan was to link the manager to the sphere scripts by this function:

itsMe.SetParent(gameObject);

but im geting this error

NullReferenceException: Object reference not set to an instance of an object

refering to the line mentioned above.

please help

thanks ahead.

Answer 1

You can not explicitly cast an Object as a Component

Instantiate() returns an Object, where you state return as SphereObj. Your attempting to return an Object as a SphereObj, which is not allowed. It is also important to note that SphereObj is derived from your newly created object, despite the fact that it is a component. For this to work, you must ensure that the game object referenced with preSphere contains a SphereObj component.

Use GetComponent<>() to retrieve a Component from a GameObject

Instead, try explicitly casting the return as a GameObject, and appending a GetComponent<> inline.

SphereObj itsMe = 
        (Instantiate(preSphere, new Vector3(0, 0, 0), Quaternion.identity) 
        as GameObject).GetComponent<SphereObj>();

itsMe.SetParent(gameObject);

---

By explicitly returning the game reference as a GameObject, you are able to retrieve the SphereObj component, and set it up as intended.

---

Further Thoughts

You describe setting up a "sphere manager", in a context where there would only logically be one "sphere manager". Perhaps it might be easier, in the long run, to set this up as a singleton-styled script.

class SphereManager
{
    public static SphereManager selfReference;

    void OnStart()
    {
        selfReference = this;
    }
}

In the above example, I create a class that holds a static self reference. This self reference sets itself up during OnStart(), but presuming I only ever have one instance of SphereManager, I do not have an issue with future instances potentially breaking my reference.

I can now call SphereManager.selfReference from anywhere in my code, to access this single instance I have running in my scene. Without having to use the SetParent() function in your SphereObj, you could still call SphereManager.selfReference to call to the manager. You could also access it from other classes, if you need to perform any other logic that involves your spheres.