Difference between Lookup() and Dictionary(Of list())

Question

I'm trying to wrap my head around which data structures are the most efficient and when / where to use which ones.

Now, it could be that I simply just don't understand the structures well enough, but how is an ILookup(of key, ...) different from a Dictionary(of key, list(of ...))?

Also where would I want to use an ILookup and where would it be more efficient in terms of program speed / memory / data accessing, etc?

Answer 1

Two significant differences:

• Lookup is immutable. Yay :) (At least, I believe the concrete Lookup class is immutable, and the ILookup interface doesn't provide any mutating members. There could be other mutable implementations, of course.)
• When you lookup a key which isn't present in a lookup, you get an empty sequence back instead of a KeyNotFoundException. (Hence there's no TryGetValue, AFAICR.)

They're likely to be equivalent in efficiency - the lookup may well use a Dictionary<TKey, GroupingImplementation<TValue>> behind the scenes, for example. Choose between them based on your requirements. Personally I find that the lookup is usually a better fit than a Dictionary<TKey, List<TValue>>, mostly due to the first two points above.

Note that as an implementation detail, the concrete implementation of IGrouping<,> which is used for the values implements IList<TValue>, which means that it's efficient to use with Count(), ElementAt() etc.