c# - An integer array as a key for Dictionary

Question

I wish to have the dictionary which uses an array of integers as keys, and if the integer array has the same value (even different object instance), they will be treated as the same key. How should I do it?

The following code does not work as b is different object instances.

 int[] a = new int[] { 1, 2, 3 };
 int[] b = new int[] { 1, 2, 3 };
 Dictionary<int[], string> dic = new Dictionary<int[], string>();
 dic.Add(a, "haha");
 string output = dic[b];

Answer 1

You can create an IEqualityComparer to define how the dictionary should compare items. If the ordering of items is relevant, then something like this should work:

public class MyEqualityComparer : IEqualityComparer<int[]>
{
    public bool Equals(int[] x, int[] y)
    {
        if (x.Length != y.Length)
        {
            return false;
        }
        for (int i = 0; i < x.Length; i++)
        {
            if (x[i] != y[i])
            {
                return false;
            }
        }
        return true;
    }

    public int GetHashCode(int[] obj)
    {
        int result = 17;
        for (int i = 0; i < obj.Length; i++)
        {
            unchecked
            {
                result = result * 23 + obj[i];
            }
        }
        return result;
    }
}

Then pass it in as you create the dictionary:

Dictionary<int[], string> dic
    = new Dictionary<int[], string>(new MyEqualityComparer());

Note: calculation of hash code obtained here: What is the best algorithm for an overridden System.Object.GetHashCode?