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This is a huge head scratcher (at least, it has been for me). I literally have only one problematic line of code I've written, that checks out in Visual Studio, but starting my game immediately boots me out with a null reference exception error based on that single line no matter how I jiggle it around or configure it or rephrase it. I have a lot of experience with collections of objects in VB, and as far as I can tell, I'm not doing anything wrong, but I've never used collections of objects in C#, so.

Here is my code.

using UnityEngine;
using System.Collections;

public class CountInventory : MonoBehaviour {

    // Use this for initialization
    public static int CurrentWater = 50;
    public static int CurrentCash = 100;
    public static int ToolMode = 1;
    public static System.Collections.Generic.List<Seed> SeedBox;
    private Seed NewSeed;
    void Start ()
    {
        SeedBox.Add(NewSeed);
    }

    // Update is called once per frame
    void Update ()
    {
        GameObject.Find("GUIWater").guiText.text = "Water: "+CurrentWater;
        GameObject.Find("GUIMoney").guiText.text = "$"+CurrentCash;
    }
}

Now, another thing to note, before someone brings it up: I do have a Seed class, so I'm not trying to declare a type that doesn't exist. :) You may also ignore my Update, and Water/Cash/Toolmode variables. I just included everything for posterity.

Any time I try to do ANTHING with my SeedBox (which is declared as a list of Seeds, or it logically seems that way to me), it gives me null reference exceptions up the wazoo. I've tried all sorts of alternate codes in place of SeedBox.Add(NewSeed);. I've tried SeedBox.Add(new Seed());, I've tried directly talking to the index (by using SeedBox[0].SeedName or whichever other Seed variable)... nothing works.

Maybe I'm just not understanding how lists work in C#. Anyone have a good answer for this one?

Answer 1

0

You need to create the list before using it.

using System.Collections.Generic;
...
public static List<Seed> SeedBox = new List<Seed>();

The reason you're getting null reference exception is because SeedBox always was null for you.
This has nothing to do with lists, collections or arrays. This is C# basics. All reference type objects are null by default. Unity create instances for you when you have public (not static) variables on scripts, so if you haven't coded C# outside of unity I can imagine you are having problems wrapping your head around it. :) But this is nothing more than you hadn't created the list in the first place.

On a completely different note; you probably shouldn't call GameObject.Find every update. You can store the references during start instead:

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class CountInventory : MonoBehaviour 
{
    public static int CurrentWater = 50;
    public static int CurrentCash = 100;
    public static int ToolMode = 1;
    public static List<Seed> SeedBox = new List<Seed>();

    private Seed NewSeed;
    private GUIText water;
    private GUIText money;

    void Start ()
    {
        water = GameObject.Find("GUIWater").guiText;
        money = GameObject.Find("GUIMoney").guiText;
        SeedBox.Add(NewSeed);
    }

    void Update ()
    {
        water.text = "Water: " + CurrentWater;
        money.text = "$" + CurrentCash;
    }
}

answered Jan 07 '11 at 08:55 PM

Statement ♦♦
20.2k ● 35 ● 71 ● 176

D'oh!

In defense of my stupid mistake, it sometimes confuses me because it seems like it's out of left field whether you need to use "= new ____" or not. For instance, I don't have to use private Seed NewSeed = new Seed, it just works the way it is. I've been doing C# in Unity for a couple months now and while I have most of it figured out, the only thing I don't understand is how I know beforehand whether I need to declare something as a "new ____" or not.

Thanks though. And yeah, I forgot about GameObject.Find on Update... this is month-old code I wrote and just came back to today. :)

Jan 07 '11 at 09:09 PM Jason B

Well, NewSeed is bound to be default(NewSeed), which is null if it is a class or default values if it is a struct. That's basically the rule there is to it. If it is a struct (value type) you dont need to new it, if it is a class (reference type) you need to new it (unless you handle nulls in an expected manner).

Jan 07 '11 at 09:13 PM Statement ♦♦

...I also laughed out loud at myself when I realized System.Collections is already included and I typed it out in the longest, most redundant way possible. Good times. :P

Jan 07 '11 at 09:13 PM Jason B

Default values for structs are 0, false and null for all kinds of members. You can't have explicit default constructors with structs. If you do try to create default constructors on structs you get the following message: "Structs cannot contain explicit parameterless constructors". Likewise you can't set struct field initializers.

Jan 07 '11 at 09:15 PM Statement ♦♦

No it wasn't. You had System.Collections but the list is in System.Collections.Generic.

Jan 07 '11 at 09:15 PM Statement ♦♦

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Null reference exception when using lists of my object.

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