Alex Alex - 2 months ago 21
C# Question

Unity: Null while making new class instance

I got stuck in pretty dumb situation: I'm making new instance of the generic class but it returns "weird" null.

Rule rule2 = new Rule(); // initiate the class
Debug.Log(rule2); //1st debug
rule2.RuleSetup(r: "CaughtEnough", li: 0); //setting up the parameters
Debug.Log(rule2.rule); //2nd debug


1st debug gives me

null
UnityEngine.Debug:Log(Object)


at the same time setting up the parameters works, and 2nd debug gives me

CaughtEnough
UnityEngine.Debug:Log(Object)


which is what supposed to be in the proper class instance.

One (only so far) issue that it is bringing to me is that if whitin this Rule class instance I call

Invoke(rule, 0f);


it gives me the NullReferenceException error. But at the same time the actual function

CaughtEnough();


works just fine and as expected.

Any ideas what could be the source of the problem and how to overcome it?

UPD also posting setup part of Rule class, as asked, though it is straightforward

public class Rule : MonoBehaviour {

public string rule;

public int leftInt;
public Dictionary<string, int> leftDict;
public float countdown;

public int outcome;

public CatchManager catchMan;
public Net net;

// Use this for initialization
void Start () {
RuleSetup();
}

public void RuleSetup(string r = "NoRule", int li = 0, Dictionary<string, int> ld = null, float cd = float.PositiveInfinity) {
rule = r;
leftInt = li;
leftDict = ld;
countdown = cd;
}
.....

Answer Source
public class Rule : MonoBehaviour{}
Rule rule2 = new Rule();

You can't use new keyword to create new instance if you are inheriting from MonoBehaviour.

You should get exception that says:

You are trying to create a MonoBehaviour using the 'new' keyword. This is not allowed. MonoBehaviours can only be added using AddComponent(). Alternatively, your script can inherit from ScriptableObject or no base class at all

Your code would have worked if you had public class Rule {} but you have public class Rule : MonoBehaviour {}.

Once you inherit from MonoBehaviour, you should either use GameObject.AddComponent or Instantiate.

Rule rule2 = null;
void Start()
{
  rule2 = gameObject.AddComponent<Rule>();
}

OR

public Rule rulePrefab;
Rule rule2;
void Start()
{
    rule2 = Instantiate(rulePrefab) as Rule;
}

If the Rule script already exist and is attached to the GameObject, you don't need to create/add/instantiate new instance of that script. Just use GetComponent function to get the script instance from the GameObject it is attached to.

Rule rule2;
void Start()
{
    rule2 = GameObject.Find("NameObjectScriptIsAttachedTo").GetComponent<Rule>();
}

EDIT:

In the latest version of Unity, creating new instance of a script that inherits from MonoBehaviour with the new keyword may not give you error and may not be null too but all the callback functions will not execute. These includes the Awake, Start, Update functions and others. So, you still have to do it properly as mentioned at the top of this answer.