XNA Difference between List and List[] when you try to create them

Question

So, I have a list of vectors, when I put a new vector in it it works perfectly, but If I try to access an index of List of Vectors and put a vector in it, I get this error "Object reference not set to an instance of an object." The code is almost the same for each List:

class GameMap
{
    MouseHandler mouseHandler;
    TileSet tileSet;
    List<Vector2>[] tiles;
    List<Vector2> tile;

    public GameMap(ContentManager Content)
    {
        mouseHandler = new MouseHandler();
        tileSet = new TileSet(Content);
    }

    public void Initialize()
    {
        tiles = new List<Vector2>[tileSet.tiles]; //What am I doing wrong here?
        tile = new List<Vector2>();
    }

    public void MapEditor()
    {
        mouseHandler.MouseUpdate();

        if (mouseHandler.LeftButton == true)
        {
            tiles[0].Add(mouseHandler.MousePosition); //The error comes out here.
            tile.Add(mouseHandler.MousePosition);
        }
    }

    public void Draw(SpriteBatch spriteBatch)
    {
        for (int i = 0; i < tileSet.tiles; i++)
        {
            tileSet.TiledTexture(spriteBatch, tiles[i], i);
            tileSet.TiledTexture(spriteBatch, tile, i);
        }
    }
}

The "Tiles" with multiples Lists of vectors isn't working, what am I doing wrong?

score 2 · Accepted Answer · answered Sep 18 '11 at 21:22

tiles = new List<Vector2>[tileSet.tiles]

creates a new array of List<Vector2> with tileSet.tiles elements. Those elements are null initially. When you access tiles[0].Add(...), tiles[0] is still null, and thus you get a null reference exception.

You need to do something similar to this:

tiles = new List<Vector2>[tileSet.tiles]
for (int i = 0; i < tiles.Length; ++i)
{
     tiles[i] = new List<Vector2>();
}

EDIT:

I guess the point is this:

Although

tile = new List<Vector2>()

and

tiles = new List<Vector2>[tileSet.tiles]

look very similar, they are quite different. The first creates a new List, and the second creates an array of List references, which are initially null. You still have to initialize each of those references to a valid list before you can use it.

score 1 · Answer 2 · answered Sep 18 '11 at 21:20

1

tiles[0].Add(mouseHandler.MousePosition);

Although you've created an array to contain a bunch of Lists, You have to instantiate each List in that array.

//something like this:
tiles[0] = new List<Vector2>();
tiles[1] = new List<Vector2>();
...
tiles[n] = new List<Vector2>();

What are you trying to contain with an array of lists? Possibly there is a less nested/complex way to accomplish your goal.

answered Sep 18 '11 at 21:20

Steve H

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I have a tileset with different tiles and I need to draw each of the tiles in some positions, So I use the first List of position for TileA(grass) the second List of position for TileB(ground), etc. – Nairda Sep 18 '11 at 21:49
@Nairda: In that case, as a first step, I would define an `enum TileSetType { Grass, Ground, ...}` and use a `Dictionary>` instead of an array. You can then do this: `tiles[TileSetType.Grass].Add(...)`, which shows your intent much clearer. – TheFogger Sep 19 '11 at 08:57

XNA Difference between List and List[] when you try to create them

2 Answers2