Null reference when trying to return an object

Questions & Answers
1

I’ve been trying to break apart a simple pathfinder I built earlier into a few smaller classes in separate script files from one big file. However, I keep running into this null reference exception and can’t figure out what’s causing it for the life of me.

I have a Node class which stores basic information about position and whether the node can be traveled:

public class Node
{
	public Vector3 ID;
	public int cost = 0, heuristic = 0, total = 0;
	public bool walkable;
	
	public Node (Vector3 ID, bool walkable)
	{
		this.ID = ID;
		this.walkable = walkable;
	}

	public void SetWalkable (bool walkable) {
		this.walkable = walkable;
	}
}

And a NodeGrid class which has a simple constructor to build a 2D array of Nodes and some methods for returning a specific node.

public class NodeGrid
{
	private Node[,] grid;

	public NodeGrid (int width, int height) {
		bool walkable = true;
		Vector3 ID;

		Node[,] grid = new Node[width, height];
		
		for (int i=0; i < width; i++)
		{
			for (int j=0; j < height; j++)
			{				
				ID.x = i;
				ID.y = 0;
				ID.z = j;
				grid[i,j] = new Node(ID, walkable);
			}
		}

		Debug.Log(width.ToString() + ", " + height.ToString() + " grid created.");
        Debug.Log(grid[5,5].ID.x.ToString());
	}

	public Node GetNode (Vector3 ID) {
		return grid[(int)ID.x, (int)ID.z]; 
	}

	public Node GetNode (int i, int j) {
		return grid[i,j]; 
	}
}

And I have a small script attached to an empty gameobject for testing this grid of nodes. It creates a new 10x10 NodeGrid and then tries to get the node at 5,5 and instead I get a null reference exception when trying to use the GetNode method.

public class NodeGridTester : MonoBehaviour {

	private NodeGrid nodeGrid;
	private int width = 10, height = 10;

	void OnGUI ()
	{
		if(GUI.Button( new Rect( 10, 10, 100, 30 ), "Create Grid")) {
			nodeGrid = new NodeGrid(width, height);
		}

		if(GUI.Button( new Rect( 10, 45, 100, 30 ), "Display Grid" )) {
			Node node = nodeGrid.GetNode(5,5);
		}
	}
}

There are no errors on compile. The runtime error message:

NullReferenceException: Object reference not set to an instance of an object
NodeGrid.GetNode (Int32 i, Int32 j) (at Assets/_TBS Assets/Scripts/NodeGrid.cs:33)

The line reference is

return grid[i,j];

from the GetNode method.

The grid appears to build just fine when using the Create Grid button, the debug text I have inserted at the end of the grid building loops shows exactly what it should.

10, 10 grid created.
UnityEngine.Debug:Log(Object)
NodeGrid:.ctor(Int32, Int32) (at Assets/_TBS Assets/Scripts/NodeGrid.cs:25)
NodeGridTester:OnGUI() (at Assets/_TBS Assets/Scripts/NodeGridTester.cs:12)

And the second debug line displays 5, reading the x of the Vector3 ID of the Node instance exactly as expected.

5
UnityEngine.Debug:Log(Object)
NodeGrid:.ctor(Int32, Int32) (at Assets/_TBS Assets/Scripts/NodeGrid.cs:26)
NodeGridTester:OnGUI() (at Assets/_TBS Assets/Scripts/NodeGridTester.cs:12)

However, pressing the Display Grid button results in the null reference.

Clearly I’m doing something wrong when it comes to creating or handling or reading from classes in C# or Unity (or maybe in general?). I looked up a bunch of topics on null reference exceptions but it’s such a broad error that nothing I found was relevant or had any effect. Any help would be greatly appreciated.

3

The problem is in this line in the NodeGrid constructor…

Node[,] grid = new Node[width, height];

You’re creating a local version of the variable since you’re specifying the type, so the class member grid is left as null. Just change it to this…

grid = new Node[width, height];

That will access the grid variable at the class scope and you should be good to go.

2

You have to create your Vector3 ID object…

public NodeGrid (int width, int height) {
       bool walkable = true;
       Vector3 ID = Vector3.zero;
...

or

...

ID = new Vector3(i,0,j);
grid[i,j] = new Node(ID, walkable);

...