What is the difference between LINQ ToDictionary and ToLookup? They seem to do the same thing.
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A dictionary is a 1:1 map (each key is mapped to a single value), and a dictionary is mutable (editable) after the fact.
A lookup is a 1:many map (multi-map; each key is mapped to an IEnumerable<> of the values with that key), and there is no mutate on the ILookup<,> interface.
As a side note, you can query a lookup (via the indexer) on a key that doesn't exist, and you'll get an empty sequence. Do the same with a dictionary and you'll get an exception.
So: how many records share each key?
An overly simplified way of looking at it is that a Lookup<TKey,TValue> is roughly comparable to a Dictionary<TKey,IEnumerable<TValue>>