使用LINQ将IEnumerable映射到ILookup的紧凑方法

Question

A situation I've found myself in a few times: I have an IEnumerable<T> , and a function map that maps an instance of T to an IEnumerable<U> . I want to produce an ILookup<T, U> defined by that map.

This seems like a pretty common use case, but I haven't found a compact way to produce the lookup. In particular, enumT.ToLookup( t=>t, t=>map(t)) produces an ILookup<T, IEnumerable<U>> .

Answer 1

Simply use SelectMany to flatten the sequence out before calling ToLookup .

var lookup = sequence.SelectMany(key => Foo(key), (key, value) => new { key, value, })
    .ToLookup(pair => pair.key, pair => pair.value);