Question

I try to write a program where Dictionary is indexed by List. (trust me i do, and yes there are option, but i like indexing by list). There is a minimal working (actually not working, only one last line which is a problem) example:

using System;
using System.Collections.Generic;

namespace test
{
    class Program
    {
    static void Main(string[] args)
    {
        Dictionary<List<String>, int> h = new Dictionary<List<string>,int>();

        List<String> w = new List<string> {"a"};
        h.Add(w, 1);

        w = new List<string>{"b"};
        h.Add(w,2);

        w = new List<string>{"a"};

        int value = 0;
        h.TryGetValue(w, out value);
        Console.WriteLine(value+" "+h[w]);
    }
}

if one debugs this program, he will clearly see that there two elements in h, but still these elements are not accessible via correct indexes — h[w]. Am I wrong or is there something weird going on?

Answer 1

The problem with your app extends from the fact that:

new List<String> { "a" } != new List<String> { "a" }

Equality for lists checks to see if the two references refer to the same instance. In this case, they don’t. You’ve instead created two Lists with the same elements…which doesn’t make them equal.

You can fix the problem by creating a custom Equality Comparer:

public class ListEqualityComparer<T> : IEqualityComparer<List<T>>
{
    public bool Equals(List<T> list1, List<T> list2)
    {
        return list1.SequenceEquals(list2);
    }

    public int GetHashCode(List<T> list)
    {
        if(list != null && list.Length > 0)
        {
            var hashcode = list[0].GetHashCode();
            for(var i = 1; i <= list.Length; i++)
                hashcode ^= list[i].GetHashCode();

            return hashcode;
        }

        return 0;
    }
}

And then passing that to the Dictionary constructor:

Dictionary<List<String>, int> h = 
    new Dictionary<List<string>,int>(new ListEqualityComparer<String>());