C# Unable to create array of objects

Question

I'm trying to create a 2D array of objects (basically an XY coordinate system), but I'm not sure how. I have a Map class which creates Tile objects. In the constructor for the Map class, I have written code to create a 2D jagged array of Tile objects.

I'm not sure why this isn't working, previously I had created 2D jagged arrays of integers and that was working fine.

What's causing the error and how should I be trying to create the array of objects?

This is the error I'm getting:

Unhandled Exception: System.NullReferenceException: Object reference not set to
an instance of an object.
   at ObjectArray.Map..ctor(Int32 NumberOfRows, Int32 NumberOfColumns) in C:\Use
rs\Lloyd\documents\visual studio 2010\Projects\ObjectArray\ObjectArray\Map.cs:li
ne 27
   at ObjectArray.Program.Main(String[] args) in C:\Users\Lloyd\documents\visual
 studio 2010\Projects\ObjectArray\ObjectArray\Program.cs:line 18

My Tile.cs

class Tile
{
    public int TileID { get; set; }

}

And my Map.cs:

class Map
{
    private Tile[][] TileGrid;

    public int Columns { get; private set; }
    public int Rows { get; private set; }

    public Map(int NumberOfRows, int NumberOfColumns)
    {
        Rows = NumberOfRows;
        Columns = NumberOfColumns;


        TileGrid = new Tile[NumberOfRows][];
        for (int x = 0; x < TileGrid.Length; x++)
        {
            TileGrid[x] = new Tile[NumberOfColumns];
        }

        //Test for the right value.
        TileGrid[0][0].TileID = 5;
        Console.WriteLine(TileGrid[0][0].TileID);

    }
}

Andrei · Accepted Answer · 2013-08-21 13:11:05Z

5

This line

TileGrid[x] = new Tile[NumberOfColumns];

creates an array of null references of the given length. So you need to iterate over it and initialize each reference with an object:

TileGrid = new Tile[NumberOfRows][];
for (int x = 0; x < TileGrid.Length; x++)
{
    TileGrid[x] = new Tile[NumberOfColumns];
    for (int y = 0; y < TileGrid[x].Length; y++)
    {
        TileGrid[x][y] = new Tile();
    }
}

answered Aug 21, 2013 at 13:11

Andrei

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Comments

Hogan · Accepted Answer · 2013-08-21 13:13:43Z

0

Before you can use an element you must make a new one... like this:

TileGrid[0][0] = new Tile();

Then you can use it:

TileGrid[0][0].TileID = 5;

The error messages is exactly true, you were trying to reference a null Tile location.

answered Aug 21, 2013 at 13:13

Hogan

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