Getting keys from a Lookup

Question

How do I get the collection of keys from a Lookup<> I created through the .ToLookup() method?

I have a lookup which maps int-values to groups of instances of a custom class. I need a collection of all the int keys that the lookup contains. Any way to do this, or do I have to collect and save them separately?

score 72 · Accepted Answer · answered Apr 26 '11 at 14:27

72

You can iterate through the set of key-item groups and read off the keys, e.g.

var keys = myLookup.Select(g => g.Key).ToList();

answered Apr 26 '11 at 14:27

Rup

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Any theory as to why `Lookup` doesn't include `Keys` method unlike `Dictionary`? – NetMage Sep 14 '17 at 22:04
I don't know - best guess maybe because Lookup is provided by LINQ and you can use LINQ to extract the keys like this? Here are the current implementations from the .NET reference source: [Lookup](https://referencesource.microsoft.com/#System.Core/System/Linq/Enumerable.cs,cb695d4a973ef608), [Dictionary](https://referencesource.microsoft.com/#mscorlib/system/collections/generic/dictionary.cs,d3599058f8d79be0). Their internal storage looks roughly the same. Dictionary has its own KeyCollection class, which would be easy enough to adapt for Lookup as well but isn't just compatible as-is. – Rup Sep 15 '17 at 19:11
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Maybe it's relevant that, unlike dictionaries, the lookup doesn't enforce only a fixed set of keys. You can query on any key you like and you get a valid response. It may return an empty set, but it is still a valid response. – Tormod Aug 20 '20 at 08:59

score 15 · Answer 2 · answered Apr 26 '11 at 14:30

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One fast way:

var myKeys = myLookup.Select(l=>l.Key);

answered Apr 26 '11 at 14:30

KeithS

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This works if you only need an IEnumerable - not a List – KevinVictor Mar 04 '21 at 17:16
@KevinVictor add a `.ToList()` to the method chain and you have a list. Not difficult. – KeithS Aug 10 '23 at 19:00

Getting keys from a Lookup

2 Answers2