What is the best way to store a map of key object to a collection of value objects?
I could use
Dictionary<KeyType, Collection<ValueType>>
but that approach tends to involve a lot of housekeeping, and clutters up the code.
The Lookup type is almost the result I'm looking for, but it's immutable and has no constructor. I could create my own custom class, but it seems like it should exist in the framework somewhere.
That's about your best answer. It's pretty common to use Dictionary<KeyType, ICollection<ValueType>>. Using the var keyword to declare your local variables is the best way to tidy up.
Other than that, your only option is creating a MultiDictionary<T,K> type. You could also call it Lookup and put it in your own namespace?
Edit: Google says you should take a look at this for an existing implementation: Multi-value Dictionary C# source code (.NET 3.5). BSD2 license means you can use it for commercial apps with at attribution. :)
You could also, as for tidying, alias the specific generic type:
using MultiDictionary = Dictionary<string, ICollection<int>>;
Then later:
var foo = new MultiDictionary();
In .NET 3.5, there is ILookup<Tkey,TValue> and Lookup<TKey,TValue> that serves this purpose; however, the default implementation is immutable. I wrote a mutable variant for MiscUtil; EditableLookup<TKey,TValue> - which does exactly what you want.
