Is there a generic collection with a key/value pair where key can occur more than once?

Question

I want to use a generic collection like Dictionary, but Dictionary requires that every key be unique. I have multiple values for the same "key", so I need a generic collection that will allow for that.

I realize that this makes the key no longer really a key, but I don't know what else to call it.

Answer 1

Several options for you to consider:

• use a Dictionary<TKey, List<TValue>> — keep a list of values for each key, not preventing duplicate values for the same key (i.e. duplicate pairs);
• use a Dictionary<TKey, HashSet<TValue>> — keep a set of value for each key, preventing duplicate values for the same key;
• use a List<KeyValuePair<TKey, TValue>> — keep a list of pair, not preventing duplicate values for the same key.

Note that in the latter case KeyValuePair is a struct, not a class, hence that implies a bit different handling.

The right option depends on your actual use case.

Answer 2

In .NET 3.5 and above, that is ILookup<TKey,TValue>. Unfortunately the only provided implementation is the immutable Lookup<TKey,TValue>, however it is easy to re-implement. An EditableLookup<TKey,TValue> is included in MiscUtil.

With an ILookup<TKey,TValue>, the TKey indexer returns an IEnumerable<TValue> (even if there were no matches for that key), so typical usage is:

foreach(var value in lookup[key])
    DoSomethingWith(value);