Multi-key dictionaries (of another kind) in C#?

Question

Building on this question, is there a simple solution for having a multi-key dictionary where either key individually can be used to identify the value?

ie.

MultikeyDictionary<TKey1, TKey2, TValue> foo;
foo.Add(key1, key2, value);
myValue = foo[key1];
// value == myValue
foo.Remove(key2);
myValue = foo[key1]; // invalid, Exception or null returned

Answer 1

Sure, it's an OO language and you can implement whatever O's you want. You are going to have some ambiguity to resolve (what if TKey1 and TKey2 are the same type, which methods get called then?)

Answer 2

You won't be able to define the overloads for both types, and the generics system doesn't allow for an arbitrary number of types (like methods allow params). So, you'd be stuck with a set of classes which defined 2, 3, 4, etc. simultaneous keys. Additionally, you'd have to use object as the parameter for get and set, using runtime type checks to simulate the overload.

Additionally, you'd only store one dictionary of <TKEY1,VAL>, the other dictionaries would be of <TKEY2,TKEY1>, <TKEY3,TKEY1> and would act as indexes on the main dictionary.

It's mostly boiler plate code.

Answer 3

I find many answers here unnecessarily complex, less performant or plain unusable. The best approach would be to have a KeyValuePair<> of the secondary key and the value clubbed together as the Value of either dictionaries. This lets you have just one lookup for for removal and updation operations. A straightforward implementation:

public class DualDictionary<TKey1, TKey2, TValue> : IEnumerable<KeyValuePair<Tuple<TKey1, TKey2>, TValue>>
{
    Dictionary<TKey1, KeyValuePair<TKey2, TValue>> _firstKeys;
    Dictionary<TKey2, KeyValuePair<TKey1, TValue>> _secondKeys;

    public int Count
    {
        get
        {
            if (_firstKeys.Count != _secondKeys.Count)
                throw new Exception("somewhere logic went wrong and your data got corrupt");

            return _firstKeys.Count;
        }
    }

    public ICollection<TKey1> Key1s
    {
        get { return _firstKeys.Keys; }
    }

    public ICollection<TKey2> Key2s
    {
        get { return _secondKeys.Keys; }
    }

    public IEnumerable<TValue> Values
    {
        get { return this.Select(kvp => kvp.Value); }
    }

    public DualDictionary(IEqualityComparer<TKey1> comparer1 = null, IEqualityComparer<TKey2> comparer2 = null)
    {
        _firstKeys = new Dictionary<TKey1, KeyValuePair<TKey2, TValue>>(comparer1);
        _secondKeys = new Dictionary<TKey2, KeyValuePair<TKey1, TValue>>(comparer2);
    }



    public bool ContainsKey1(TKey1 key)
    {
        return ContainsKey(key, _firstKeys);
    }

    private static bool ContainsKey<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> dict)
    {
        return dict.ContainsKey(key);
    }

    public bool ContainsKey2(TKey2 key)
    {
        return ContainsKey(key, _secondKeys);
    }

    public TValue GetValueByKey1(TKey1 key)
    {
        return GetValueByKey(key, _firstKeys);
    }

    private static TValue GetValueByKey<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> dict)
    {
        return dict[key].Value;
    }

    public TValue GetValueByKey2(TKey2 key)
    {
        return GetValueByKey(key, _secondKeys);
    }

    public bool TryGetValueByKey1(TKey1 key, out TValue value)
    {
        return TryGetValueByKey(key, _firstKeys, out value);
    }

    private static bool TryGetValueByKey<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> dict, out TValue value)
    {
        KeyValuePair<T, TValue> otherPairing;
        bool b = TryGetValue(key, dict, out otherPairing);
        value = otherPairing.Value;
        return b;
    }

    private static bool TryGetValue<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> dict,
                                          out KeyValuePair<T, TValue> otherPairing)
    {
        return dict.TryGetValue(key, out otherPairing);
    }

    public bool TryGetValueByKey2(TKey2 key, out TValue value)
    {
        return TryGetValueByKey(key, _secondKeys, out value);
    }

    public bool Add(TKey1 key1, TKey2 key2, TValue value)
    {
        if (ContainsKey1(key1) || ContainsKey2(key2))   // very important
            return false;

        AddOrUpdate(key1, key2, value);
        return true;
    }

    // dont make this public; a dangerous method used cautiously in this class
    private void AddOrUpdate(TKey1 key1, TKey2 key2, TValue value)
    {
        _firstKeys[key1] = new KeyValuePair<TKey2, TValue>(key2, value);
        _secondKeys[key2] = new KeyValuePair<TKey1, TValue>(key1, value);
    }

    public bool UpdateKey1(TKey1 oldKey, TKey1 newKey)
    {
        return UpdateKey(oldKey, _firstKeys, newKey, (key1, key2, value) => AddOrUpdate(key1, key2, value));
    }

    private static bool UpdateKey<S, T>(S oldKey, Dictionary<S, KeyValuePair<T, TValue>> dict, S newKey,
                                        Action<S, T, TValue> updater)
    {
        KeyValuePair<T, TValue> otherPairing;
        if (!TryGetValue(oldKey, dict, out otherPairing) || ContainsKey(newKey, dict))
            return false;

        Remove(oldKey, dict);
        updater(newKey, otherPairing.Key, otherPairing.Value);
        return true;
    }

    public bool UpdateKey2(TKey2 oldKey, TKey2 newKey)
    {
        return UpdateKey(oldKey, _secondKeys, newKey, (key1, key2, value) => AddOrUpdate(key2, key1, value));
    }

    public bool UpdateByKey1(TKey1 key, TValue value)
    {
        return UpdateByKey(key, _firstKeys, (key1, key2) => AddOrUpdate(key1, key2, value));
    }

    private static bool UpdateByKey<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> dict, Action<S, T> updater)
    {
        KeyValuePair<T, TValue> otherPairing;
        if (!TryGetValue(key, dict, out otherPairing))
            return false;

        updater(key, otherPairing.Key);
        return true;
    }

    public bool UpdateByKey2(TKey2 key, TValue value)
    {
        return UpdateByKey(key, _secondKeys, (key1, key2) => AddOrUpdate(key2, key1, value));
    }

    public bool RemoveByKey1(TKey1 key)
    {
        return RemoveByKey(key, _firstKeys, _secondKeys);
    }

    private static bool RemoveByKey<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> keyDict,
                                          Dictionary<T, KeyValuePair<S, TValue>> valueDict)
    {
        KeyValuePair<T, TValue> otherPairing;
        if (!TryGetValue(key, keyDict, out otherPairing))
            return false;

        if (!Remove(key, keyDict) || !Remove(otherPairing.Key, valueDict))
            throw new Exception("somewhere logic went wrong and your data got corrupt");

        return true;
    }

    private static bool Remove<S, T>(S key, Dictionary<S, KeyValuePair<T, TValue>> dict)
    {
        return dict.Remove(key);
    }

    public bool RemoveByKey2(TKey2 key)
    {
        return RemoveByKey(key, _secondKeys, _firstKeys);
    }

    public void Clear()
    {
        _firstKeys.Clear();
        _secondKeys.Clear();
    }

    public IEnumerator<KeyValuePair<Tuple<TKey1, TKey2>, TValue>> GetEnumerator()
    {
        if (_firstKeys.Count != _secondKeys.Count)
            throw new Exception("somewhere logic went wrong and your data got corrupt");

        return _firstKeys.Select(kvp => new KeyValuePair<Tuple<TKey1, TKey2>, TValue>(Tuple.Create(kvp.Key, kvp.Value.Key),
                                                                                      kvp.Value.Value)).GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Few things to note:

1. I have implemented only IEnumerable<>. I don't think ICollection<> makes sense here since the method names all could be way different for this special collection structure. Up to you to decide what should go inside IEnumerable<>.

2. I have attempted for some weird exceptions to be thrown here and there - just for data integrity. Just to be on the safer side so that you know if ever my code has bugs.

3. I have named methods in such a way that its compilable even when Key1 and Key2 are of the same type.

4. Performance: You can lookup for Value with either of the Keys. Get and Contains method require just 1 lookup (O(1)). Add requires 2 lookups and 2 adds. Update requires 1 lookup and 2 adds. Remove takes 3 lookups.