Beginner topic: Trying to Understand a script error ~NULL REFERENCE EXCEPTION(SOLVED)

THE CODE

Spoiler

Code (csharp):

1. using UnityEngine;
2.  
3. [System.Serializable]
4. public class ColorToPrefab {
5.  
6.     public Color32 color;
7.     public GameObject prefab;
8.     public int index;
9.     public string name;
10. }

Code (csharp):

1. using UnityEngine;
2.  
3. public class LevelGenerator : MonoBehaviour {
4.  
5.     public Texture2D map;
6.  
7.     public ColorToPrefab[] colorMappings;
8.  
9.     GameObject[,]prefabTileMap;
10.  
11.     void Start () {
12.         GenerateLevel();
13.     }
14.  
15.     void GenerateLevel ()
16.     {
17.         Color32[] pColor = map.GetPixels32 ();
18.         int w = map.width;
19.         int h = map.height;
20.         for (int x = 0; x < w; x++)
21.         {
22.             for (int y = 0; y < h; y++)
23.             {
24.                 Debug.Log ("* * * * *   GeneratingTile --> x: " + x + ", y: " + y+"    * * * * *");
25.                 int index = x + y * w;
26.                 GenerateTile(pColor[index],x, y);
27.             }
28.         }
29.     }
30.  
31.     void GenerateTile (Color32 color, int x, int y)
32.     {
33.         Debug.Log ("c: "+color);
34.         if (color.a == 0) {
35.             // The pixel is a path. Let's ignore it!
36.             Debug.Log("PATH FOUND -RETURN-");
37.             prefabTileMap [x, y] = null;
38.             return;
39.         }
40.  
41.         foreach (ColorToPrefab colorMapping in colorMappings)
42.         {
43.             Debug.Log ("ColorMapping.color: "+colorMapping.prefab.name);
44.             if (colorMapping.color.Equals(color))
45.             {
46.                 Debug.Log ("ColorMapping.color == pixelColor --> CREATING OBJECT");
47.                 Vector3 position = new Vector3(x, 1, y);
48.                 GameObject go = (GameObject)Instantiate (colorMapping.prefab, position, Quaternion.identity, transform);
49.  
50.                 prefabTileMap [x, y] = go; // THIS IS WHERE ERROR OCCURS
51.                 GridIndex gi = go.GetComponent<GridIndex> ();
52.                 gi.indexX = x;
53.                 gi.indexY = y;
54.                 return;
55.             }
56.         }
57.     }
58.  
59. }
60.  

THE ERROR

DESCRIPTION:
I'm trying to understand why I got the error. The list ColorToPrefab has a color, and a prefab tied to it as you can see in the code above.

 

Spoiler: Info on reference vs value types

In C# you have two data types, a reference type and a value type.

Classes (like GameObject) are reference types, structs (Vector3, Color) and primitives (int, double, float) are value types.

When you pass a value type into a method, you give it the value, not the object itself. The value of an int will be copied and given to the method.

With reference types, a variable storing one points to the object in memory (usually on the heap). When you give it to a method you give the method a pointer to the object, so both the caller and the callee are using the exact same object.

This also means a reference type can be null, ie pointing to nothing. A NRE is when you try to use a value from an object that doesn't exist.

First check what value is null so you can find out why it is. Just put:

Code (CSharp):

1. print(prefabTileMap ==null ? "tilemap is null" : "tilemap is instantiated");
2. print(go == null ? "go is null" : "go is instantiated");

I'm betting on it being the tilemap not being created, I can't see an instantiation anywhere in your script.

 

Code (CSharp):

1. GameObject[,] prefabTileMap;

on line 9 is where I declare it. I assumed that the array would be initialized with "null" pointers by default. But I'm not trying to GET the value inside an array, I'm trying to SET a value in the array. I would understand the error if I were referencing a value from the array, but I'm trying to set a value to the array.

 

Okay, I see why I got the error. The prefabTileMap variable was "NULL" value. I assumed the reference variable would have been initialized and then filled with "NULL" values for it's cells. But just declaring the prefabTileMap only saved space I guess and did not assign it anything.

So, I added in

Code (CSharp):

1. prefabTileMap=new GameObject[w,h];

inside the GenerateLevel script and the code works fine. Crazy how things have to be put together.

 

You ARE referencing a value from the array. Arrays are reference types, and like I said:

In the case of a null array, you're accessing the nth element of... what? The array doesn't exist yet! Bork, throw a NRE.

Good to see you got it sorted anyway, there is a difference between declaration and assignment ;p