C# coordinate keyed dictionary

Question

I have a class Room, and a class World. Currently, I have a

    Dictionary<Point, Room> world;

which I store Rooms in like so:

    world.Add(new Point(0,0), new Room());

But when I try to access it, it returns null:

    world.Get(new Point(0,0));

I understand the reason why this is happening. But my question is: Does anybody know a better way of doing this?

Answer 1

That should work fine, provided your Point implementation implements GetHashCode and Equals properly.

For example, the following works perfectly:

using System;
using System.Collections.Generic;
using System.Drawing;

class Room
{
    public int X
    {
        get;
        set;
    }
}

struct Program
{
    static void Main()
    {
        Dictionary<Point, Room> world = new Dictionary<Point, Room>();

        world.Add(new Point(0, 0), new Room() { X = 0 });
        world.Add(new Point(2, 3), new Room() { X = 2 });

        Room room = world[new Point(2, 3)];

        Console.WriteLine(room.X);
        Console.ReadKey();
    }
}

This is using System.Drawing.Point, which implements GetHashCode properly. (It prints "2", as expected.)

I suspect the problem is your implementation of Point. Make sure it implements Equals and GetHashCode correctly, or (better yet) use a version of Point included in the framework.

Answer 2

You can provide your own IEqualityComparer when you instantiate the dictionary:

public Dictionary(IEqualityComparer<TKey> comparer)

This works even if you can't modify the original TKey class.