Get first key from Dictionary<string, string>

Question

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?

Despairing

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. – Despairing

Possible duplicate of Get first element from a dictionary – Sabah

@MichaelFreidgeim That could be a duplicate, but this question was asked 3 years before, so it couldn't really be a duplicate then. – Despairing

"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 meta.stackexchange.com/questions/147643/… If you agree that it requires clarification please vote on meta.stackexchange.com/questions/281980/… – Sabah

Answer 1

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.)

Bullington

nice, didn't see it in intellisense so i thought this was a non-linq-able object (lazy me :) – Despairing

how do you do this in .NET 2 and earlier – Philomenaphiloo

@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... – Bullington

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. – Arietta

yes, dic[dic.Keys.First()] worked great; provided dic.Keys.First()!=null. Thanks for the tips! – Hammons

@WebDrive: If you want the first value, just use dic.Values.First(). No need to go via the key. – Bullington

I got an error CS1061. Needed to add using System.Linq; to succeed. – Independence

@Jarekczek: Yes, you would. I kinda take that as read for LINQ answers :) – Bullington

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. – Aggi

Answer 2

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.

Changeful

+1 for correct. Ordering in a dictionary isn't guaranteed. Perhaps a SortedDictionary would be more useful. – Maidstone

idem dito ----------(at least 15 chars) – Despairing

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. – Isiahisiahi

Answer 3

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.

Oospore

Answer 4

Seems a bit over the top, but how about

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

Marella

Answer 5

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();

Gredel