Get first element from a dictionary

Question

I have the following declaration:

Dictionary<string, Dictionary<string, string>> like = new Dictionary<string, Dictionary<string, string>>();

I need to get the first element out, but do not know the key or value. What's the best way to do this?

Hayrack

Define "first" for a dictionary. – Additament

Dictionaries are unordered. – Soloma

Related (none other than Skeet): https://mcmap.net/q/41641/-who-39-s-on-dictionary-lt-gt-first-duplicate/1001985 – Bonita

Using First() makes sense when you know there is only one item in the dictionary because you have done a .Count on the dictionary. Then you don't need to do a foreach loop on one item, just use First() – Bigner

@LouiseEggleton This is exactly how I am using this solution. Thanks for being level headed about OP's question. – Philosophize

Could also use Single() or SingleOrDefault() instead of First(), if you know that it only has one item. – Rossi

@AustinSalonen "as the first element returned in a for each statement" – Dryer

Answer 1

Note that to call First here is actually to call a Linq extension of IEnumerable, which is implemented by Dictionary<TKey,TValue>. But for a Dictionary, "first" doesn't have a defined meaning. According to this answer, the last item added ends up being the "First" (in other words, it behaves like a Stack), but that is implementation specific, it's not the guaranteed behavior. In other words, to assume you're going to get any defined item by calling First would be to beg for trouble -- using it should be treated as akin to getting a random item from the Dictionary, as noted by Bobson below. However, sometimes this is useful, as you just need any item from the Dictionary.

---

Just use the Linq First():

var first = like.First();
string key = first.Key;
Dictionary<string,string> val = first.Value;

Note that using First on a dictionary gives you a KeyValuePair, in this case KeyValuePair<string, Dictionary<string,string>>.

---

Note also that you could derive a specific meaning from the use of First by combining it with the Linq OrderBy:

var first = like.OrderBy(kvp => kvp.Key).First();

Bonita

I've suggested an edit to change val to its proper value (since it's a string) but I guess it was rejected? – Atalanta

@Eve - Yes, because val is a Dictionary<string, string>, and like is a Dictionary<string, Dictionary<string, string>>. – Hurlee

Oh, my bad, sorry for the distraction. – Atalanta

@dbaseman A dictionary doesn't have a first pair. Just because you called First on a Dictionary doesn't mean that you actually got the first item in the dictionary. You got an item in the dictionary, but not the first item. There is no first item, because dictionaries are unordered. This type of answer only further harms the OP because they will continue to believe that they are actually getting the "first" item, even though no such thing exists. The proper answer to the question is that there is no first item. – Fibrilliform

@Fibrilliform - There is exactly one scenario I can think of where calling First on a dictionary is worth doing: When you want to access a property of the key or value objects which should be the same on all of them (i.e. a backreference to the parent in a dictionary of children). – Hurlee

@Hurlee I can think of others still. The point is that you need to realize it's not "the first item" it's "an arbitrary item". As long as you're clear what it's doing, and are okay with that, then it's fine. The OP specifically said he wanted "the first item". No such thing exists. If he asked for "an arbitrary item" then this answer would be 100% correct. – Fibrilliform

I get the error `Type 'System.Collections.Generic.Dictionary<K,V> – Letterperfect

@Hurlee it is meaningful. I have a dictionary of department (string) and department code (string). Someone in my company could work for just one department or be considered in a few. I check the count of the dictionary entries, and if there is only one then I would like that department without knowing the key or value of it. If they have more than one then I make the choose one and send that one back instead. – Philosophize

Thanks, my use case for this is implementing a DBObj.SelectFirst() method (a select method that just returns the first column of the first row of results). The nature of my method allows for me to pass that undefined behavior on to the calling function if they decide to use a query which produces more items than it should in the first place. In this case, I could use .first(), .last(), .rnd()....it doesn't matter. In practice, it always (so far) returns exactly what you wanted anyway in this case because Dictionary objects tend to keep the order they were created in. – Kravits

For others saying we're not really getting the first one - if you use .OrderBy and order it so the one you want is on top, yes, you are getting the first one. And if you just have a dictionary of one item, Iike I was doing, you are also getting the first one. – Golda

You may want to just pull any value from the collection, which is what you really are doing here. You shouldn't think of dictionaries in terms of "first" but there are still reasons you might want to just pull ONE value like i said here #1821006 – Messer

Answer 2

For anyone coming to this that wants a linq-less way to get an element from a dictionary

var d = new Dictionary<string, string>();
d.Add("a", "b");
var e = d.GetEnumerator();
e.MoveNext();
var anElement = e.Current;
// anElement/e.Current is a KeyValuePair<string,string>
// where Key = "a", Value = "b"

I'm not sure if this is implementation specific, but if your Dictionary doesn't have any elements, Current will contain a KeyValuePair<string, string> where both the key and value are null.

(I looked at the logic behind linq's First method to come up with this, and tested it via LinqPad 4)

Septime

Enumerators are IDisposable objects, so you should use using construct here. – Eightfold

Answer 3

Though you can use First(), Dictionaries do not have order per se. Please use OrderedDictionary instead. And then you can do FirstOrDefault. This way it will be meaningful.

Aiello

I don't know about "safe". "Meaningful" would be more appropriate. First will do something for a Dictionary, you just have no idea what that something is, unlike an ordered dictionary in which case First has a meaningful value. – Fibrilliform

+1, but just a thought: Not good for performance, as OrderedDictionary is non-generic, which means boxing / unboxing has to take place. – Customs

Answer 4

EDIT: Use an OrderedDictionary.

It's better to use FirstOrDefault() to retrieve the first value.

Ex:

var firstElement = like.FirstOrDefault();
string firstElementKey = firstElement.Key;
Dictinary<string,string> firstElementValue = firstElement.Value;

Breastplate

The advantage of FirstOrDefault() is that it will give a default value if there is none present in the dictionary. – Breastplate

And yet since a Dictionary is unordered, there is no such thing as a "first" item in a dictionary in the first place. No such item exists. You can get an item from the dictionary, but you need to have an ordered data structure to get the "first". – Fibrilliform

I agree. Having an orderedDictionary makes sense for "First". – Breastplate

Answer 5

using System.Linq;

Dictionary<string, Dictionary<string, string>> like = new Dictionary<string, Dictionary<string, string>>();
Dictionary<string, string> first = like.Values.First();

Dardar

.Values.First() doesn't exist... are you using System.Collection.Generic? – Gerent

What do you mean by 'doesn't exists'? Remember that First() is an extension method, so you must include using System.Linq in your using directives – Dardar

Answer 6

Dictionary does not define order of items. If you just need an item use Keys or Values properties of dictionary to pick one.

Cambyses

Answer 7

ill find easy way to find first element in Dictionary :)

 Dictionary<string, Dictionary<string, string>> like = 
 newDictionary<string,Dictionary<string, string>>();

 foreach(KeyValuePair<string, Dictionary<string, string>> _element in like)
 {
   Console.WriteLine(_element.Key); // or do something
   break;
 }

Acroterion

exactly; I hoped there were a cleaner syntax for this solution but I didn't found – Dryer

Answer 8

convert to Array

var array = like.ToArray();
var first = array[0];

Rasla

Answer 9

Easy way of to index a Collection in terms of performance, high compatibility (2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8) and easy implemention.

Save today!! Its not only a items copy, this is items reference of a Collection!! buy it!!

string [] arrayString = new string[like.Count];
like.Values.CopyTo( arrayString,0 );

arrayString[0] //First

References:

https://learn.microsoft.com/es-es/dotnet/api/system.collections.generic.icollection-1.copyto?view=net-5.0

https://social.msdn.microsoft.com/Forums/vstudio/en-US/dc5e4242-64d3-45ac-bdea-cf4f3d9abdbb/icollectioncopyto-vs-arraylisttoarray?forum=netfxbcl

Tham

While this code may answer the question, it would be better to include some context, explaining how it works and when to use it. Code-only answers are not useful in the long run. – Complicate