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I'm using a System.Collections.Generic.Dictionary<string, string>.

I want to return the first key from this dictionary. I tried dic.Keys[0] but the only thing I have on the Keys property (which is a KeyCollection object) is an enumerator.

Do I have to enumerate through all the keys to get the first one?

Amal K
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Michel
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  • as many of you have said, there is no 'first'. i had to make myself a little more clear i thing: you can specify a region in a querystring, and i check if it is in my list of regios. if it is not (user edits the querystring), i just sets it to the 'first' region which is known. – Michel Nov 30 '09 at 15:43
  • Possible duplicate of [Get first element from a dictionary](http://stackoverflow.com/questions/13979966/get-first-element-from-a-dictionary) – Michael Freidgeim Sep 04 '16 at 23:22
  • @MichaelFreidgeim That could be a duplicate, but this question was asked 3 years before, so it couldn't really be a duplicate then. – Michel Sep 05 '16 at 08:14
  • "Possible duplicate" is a way to clean-up - to close similar questions and keep one with the best answers. The date is not essential. See http://meta.stackexchange.com/questions/147643/should-i-vote-to-close-a-duplicate-question-even-though-its-much-newer-and-ha If you agree that it requires clarification please vote on http://meta.stackexchange.com/questions/281980/add-clarification-link-to-possible-duplicate-automated-comment – Michael Freidgeim Sep 05 '16 at 09:32

5 Answers5

124

Assuming you're using .NET 3.5:

dic.Keys.First();

Note that there's no guaranteed order in which key/value pairs will be iterated over in a dictionary. You may well find that in lots of cases you get out the first key that you put into the dictionaries - but you absolutely must not rely on that. As Skirwan says, there isn't really a notion of "first". Essentially the above will give you "a key from the dictionary" - that's about all that's guaranteed.

(If you don't know whether the dictionary is empty or not, you could use FirstOrDefault.)

Jon Skeet
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    nice, didn't see it in intellisense so i thought this was a non-linq-able object (lazy me :) – Michel Nov 30 '09 at 15:52
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    how do you do this in .NET 2 and earlier – ina Nov 01 '14 at 21:08
  • @ina: You could easily write a `First()` method yourself, which just uses `foreach` with a return statement in. But I'd strongly encourage you to upgrade if at *all* possible... the difference in productivity when you've got LINQ available is huge... – Jon Skeet Nov 02 '14 at 14:45
  • Thank you for this. In my situation, I just want the most convenient way to get a single key (and I don't care which.) This works great. – Danation Nov 28 '14 at 00:51
  • yes, dic[dic.Keys.First()] worked great; provided dic.Keys.First()!=null. Thanks for the tips! – WebDrive Jan 11 '16 at 16:12
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    @WebDrive: If you want the first *value*, just use `dic.Values.First()`. No need to go via the key. – Jon Skeet Jan 11 '16 at 16:36
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    I got an error CS1061. Needed to add `using System.Linq;` to succeed. – Jarekczek Mar 02 '16 at 17:56
  • @Jarekczek: Yes, you would. I kinda take that as read for LINQ answers :) – Jon Skeet Mar 02 '16 at 18:13
  • Note that there is an importance case where dic.Keys.First() is well defined - when there is just one entry in the dictionary. That was my case. – Bruce Dawson Jul 09 '19 at 21:28
23

The idea of 'first' doesn't really apply to a dictionary, as there's no ordering on the entries; any insert or remove is allowed to radically change the order of the keys.

Skirwan
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    +1 for correct. Ordering in a dictionary isn't guaranteed. Perhaps a SortedDictionary would be more useful. – Greg D Nov 30 '09 at 15:37
  • idem dito ----------(at least 15 chars) – Michel Nov 30 '09 at 15:53
  • first is of course cryptic as order is by design not to be sequential for all sorts of reasons such as security and deterring reliance on order, but there may be some situations that you would want this still. The first that comes to mind (sorry bad pun) would be when you want to pull just any old key/value out of it. Another may be, if we know there is only one element, and we just want to return it without having to add syntactical bloat to the code, which could also lead to confusion when maintenance is required on said code. – osirisgothra Aug 18 '22 at 17:33
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Firstly, no, you do not have to enumerate all of the entries — just enumerate the first item:

IEnumerator enumerator = dictionary.Keys.GetEnumerator();
enumerator.MoveNext();
object first = enumerator.Current;

Ideally one would check that the MoveNext() actually succeeded (it returns a bool) otherwise the call to Current may throw.

Please also note that the order of a dicitonary is not specified. This means that the 'first' item may in fact be any key, may be different on subsequent calls (though in practice will not) and will certainly change as items are added and removed from the dictionary.

Paul Ruane
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9

Seems a bit over the top, but how about

foreach (<Type> k in dic.Keys)
    return k;
Thorsten Dittmar
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2

we can using linq technology

var key = dic.Take(1).Select(d => d.Key).First()

or we can use another search

var myList = dic.Take(1).Where(d => d.Key.Contains("Heaven")).ToList();
Think Big
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