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I came across this class while reading a C# book and have some questions.
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LINQ ToLookup() Method ToLookup operator in LINQ is an extension method, and it is used to extract a set of key/value pairs from the source. Here, each element in the resultant collection is a generic Lookup object. Lookup object holds the Key and subsequence items that matched with the Key.
ILookUp => Group by key , Enumerable Collection. Single key value refers to enumerable collection where we can iterate through the value collection. IDictionary => Group by distinct key , Single value. Follow this answer to receive notifications.
Purpose of the class: a dictionary where a key can map to multiple values. Think of it as being for grouping rather than one-to-one mapping.
Only through ToLookup decision: Pass. Again, seems like a bad call to me. On the other hand, it means that the result is immutable to the outside world, which is quite nice. It's quite easy to write your own collection which supports this, of course - but it would be have been quite nice to have it in the collections "properly". My guess is that MS didn't have the time/money to go through the pretty rigorous design/test required to make it a first class collections decision.
Namespace decision: Probably related to the above. Having a version in System.Collections.Generic which you couldn't create yourself would have been a bit odd.
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As an aside, note that MiscUtil also includes a MiscUtil.Linq.EditableLookup<,> class, that is similar; it implements the regular ILookup<,> interface, but is fully mutable - so you can create it and add your own values.
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