Null reference exception:obj ref not set to a instance of obj

Question

This is a common question but I haven't found a solution yet to my script.

I have script b set up like this

 // Global Access
     public static Formation_AI Fai;
 
 public bool isEmptySlot1 = true;  // if the slot is empty it can be filled
     public bool isEmptySlot2 = true;  // if the slot is empty it can be filled
     public bool isEmptySlot3 = true;  // if the slot is empty it can be filled
     public bool isEmptySlot4 = true;  // if the slot is empty it can be filled
 
 above the start function

I have script A trying to use isEmptySlot1 like this

 void Update () {
         transform.LookAt(playerTarget);
 
         if (Formation_AI.Fai.isEmptySlot1)
         {
             transform.position = Vector3.Lerp(transform.position, FormTarg3.position, Time.deltaTime * smoothTranslateSpeed);     
         }

I get a null reference exception on the if statement. I don't know what is going wrong because i made public static Formation_AI Fai; in my other script and its set to a object that is in the scene. Well the object is a prefab of a root empty game object and 4 child empty game objects. But the script is on the root of the prefab.

// Current Problem -------------------------------------------// Well the prefab IS in the scene already (the fighters are supposed to move to the prefab slots in the scene), and it still gives me a null reference error. With getcomponent.

Ok the problem was I needed to set it in the inspector with a prefab with Formation_AI on it.

Answer 1

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Answer by hathol · Sep 25, 2012 at 10:16 AM

I think the prefab part is your problem. You need to create an instance of the object first before you can access it's non-static members. What exactly are you trying to do there? If you know you'll only ever need one instance of the object, have a look at the singleton pattern. Alternatively, just make the bools static as well and access them directly. Exposing a single instance of an object like that is usually not a good idea and tends to causes bad things to happen.

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cj31387 · Sep 25, 2012 at 10:27 AM 0

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There will be more than one of that prefab in the scene later on. How do I create a instance of the object? The prefab is already in the scene though. What I'm trying to do is access the variables from my formation script inside my Fighter_AI script.

hathol · Sep 25, 2012 at 10:43 AM 1

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A) You should really get rid of that global access stuff, especially if you're going to have multiple instances of that object at some point. See if you can give your Fighter object a reference to the desired Formation_AI instance directly ins$$anonymous$$d.

B) I still suspect your static variable holds only a reference to the object prefab and not to the instance in the scene. So as a short term solution: In you Formation_AI add

 void Start()
 {
     Fai = this;
 }

refardeon · Sep 25, 2012 at 11:08 AM 1

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Check out the $$anonymous$$anual here: http://docs.unity3d.com/Documentation/ScriptReference/index.Accessing_Other_Game_Objects.html

That should get you going.

refardeon · Sep 25, 2012 at 11:25 AM 1

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Yes, but like hathol said: If you use a static reference like this, you can't access more than one instance of this script. From what you wrote, I suspect that's not what you want. The problem is: $$anonymous$$ultiple instances of your script will all overwrite the static reference Fai with themselves - so in the end, only one of these scripts will be accessible by the global variable.

About GetComponent: You need an object with the Component attached somewhere where you can access it. You could also use GameObject.FindObjectOfType(); however, this is computationally expensive - don't use it every frame!. If you only need to assign the formations at the start of the game, it should suffice.

refardeon · Sep 25, 2012 at 11:37 AM 1

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Yes, if the formation prefab is actually instantiated the scene. If it's only in the project folder, GetComponent will fail.

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Null reference exception:obj ref not set to a instance of obj

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