C#: Dictionary indexed by List

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?

The list class probably doesn't produce a hashcode and equality check based on its contents. Have you checked if arrays work better? — Lasse V. Karlsen, Nov 10 '11 at 14:56

Justin Niessner · Accepted Answer · 2011-11-10T15:05:26.417

7

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>());

edited Nov 10 '11 at 15:05

answered Nov 10 '11 at 14:57

Justin Niessner

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Usually when comparing two reference types, comparison returns true if both references points to the same object. If the objects are different then even their contents are same the result will be false. – Oybek Nov 10 '11 at 15:03
Another option would be using `ToArray()` and [`IStructuralEquatable.GetHashCode`](http://msdn.microsoft.com/en-us/library/system.collections.istructuralequatable.gethashcode.aspx) for a simpler (if slower) implementation. (Or maybe using arrays and a `Dictionary` to begin with.) – millimoose Nov 10 '11 at 15:14
thanks! works like charm, it is only `for(var i = 1; i < list.Length; i++)` – Victor Ermolaev Nov 11 '11 at 09:23

JonAlb · Answer 2 · 2011-11-10T15:02:02.757

The problem is the index by List, what you are indexing by isn't the data in the list but you are essentially indexing by the memory pointer to the List (i.e the memory address of where this List is located).

You Created one list at one memory location, you then created a totally different list at a different memory location (ie when you create a new instance). The two lists are different even though they contain the same data, and this means you can add as many as you want to the dictionary.

One solution is Rather than indexing by List would be to index by String and use a comma separated List containing all the data in your list as an index.

Technically not the pointer, but this is correct. List doesn't compute hash codes based on the contents: http://ideone.com/AEIDO — millimoose, Nov 10 '11 at 15:02

score 1 · Answer 3 · answered Nov 10 '11 at 14:57

1

This won't ever work for you, because List<T>'s Equals and GetHashCode methods don't consider the contents of the list. If you want to use a collection of objects as a key, you'll need to implement your own collection type that overrides Equals in such a way as to check the equality of the objects in the collection (perhaps using Enumerable.SequenceEqual.)

answered Nov 10 '11 at 14:57

mqp

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Oh, or you can use an `IEqualityComparer`, like the fellow in the comment above mentioned, but I'll leave that to someone else. – mqp Nov 10 '11 at 15:01

score 1 · Answer 4 · answered Nov 10 '11 at 14:57

1

The Dictionary class uses reference comparison to look for the specified key, that's why even if the lists contain the same items, they are different.

answered Nov 10 '11 at 14:57

Stefan

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2

*in this specific context*. In general, it doesn't do that, but with a list, which does not override GetHashCode or Equals, it does. – Lasse V. Karlsen Nov 10 '11 at 15:00

C#: Dictionary indexed by List

4 Answers4