NullReferenceException Confusion

Questions & Answers
1

So here’s my issue:

I have a Planet class and a HexTile classes that I am using to create hexagonal (and 12 pentagonal) tiles on the surface of a sphere. My problem is coming from telling which tiles who their neighbors are.

My tiles are stored in List < HexTile > hexTiles and each tile holds its own neighbors in List < int > neighbors as an index to the hexTiles list. For some reason, when I ran my algorithm for finding each tile’s neighbors (confirmed not the issue) I got errors everywhere. I’ve narrowed it down to this most basic case.

Planet.cs

 public class Planet : MonoBehaviour {
    
    public List < GameObject > hexTiles;

    void Start () {
        hexTiles = new List < GameObject > ();

        // Icosahedron code...

        foreach ( Vector3 v in mesh.vertices ) {
            GameObject temp = new GameObject ();
            temp.AddComponent < HexTile > ();
            hexTiles.Add ( temp );
        }

        // as a test case, trying to set the first neighbor of the first tile to 42
        hexTiles [ 0 ].GetComponent < HexTile > ().neighbors.Add ( 42 ); // NullReferenceException Error

    }
}

HexTile.cs

public class HexTile : MonoBehaviour {

	public List < int > neighbors;

	void Start () {

		neighbors = new List < int > ();
		neighbors.Add ( 42 ); // works fine here

	}
}

Everything I’ve searched online has been people forgetting to initialize their List, but that I can edit the list from HexTile disproves that. If anyone has any info that could help, or know what I’m doing wrong, I would be very grateful for it!

If I need to add more info I can.

Thank you for reading!

2

Hey !
Your list neighbors is initialised in the Start method, which is called at the beginning of the next frame. However, you’re trying to access it as soon as the temp GameObject has been created, hence the NullReferenceException.
This can be solved very easily by moving your list initialisation code to its declaration, which means your HexTile scripts would become something like this :

public class HexTile : MonoBehaviour {
 
    public List < int > neighbors = new List < int > ();
 
    void Start () {

       neighbors.Add ( 42 ); // works fine here
 
    }
}