Getting a KeyValuePair<> directly from a Dictionary<>

Question

I have System.Collections.Generic.Dictionary<A, B> dict where A and B are classes, and an instance A a (where dict.ContainsKey(a) is true).

Is it possible to get the KeyValuePair containing a directly from the Dictionary?
Or do I need to create a new KeyValuePair: new KeyValuePair<A, B>(a, dict[a])?

Solution 1

You need to create a new KeyValuePair¹ - but bear in mind that KVP is a value type (a struct) anyway, so it's not like you're introducing a new inefficiency by doing this. Any method returning a KVP would be creating a copy anyway - you're just creating the instance directly.

You could always add an extension method to IDictionary<TKey, TValue> if you wanted:

public static KeyValuePair<TKey, TValue> GetEntry<TKey, TValue>
    (this IDictionary<TKey, TValue> dictionary,
     TKey key)
{
    return new KeyValuePair<TKey, TValue>(key, dictionary[key]);
}

As noted in comments, it's entirely possible that the key which is stored in the dictionary is not the same as the one provided, just semantically equal - by some semantics which could be customized by an IEqualityComparer (as with a case-insensitive dictionary, for example.) In that case, the code above would not return the actual entry in the dictionary, but an entry with the key you provided to look up. Unfortunately there's no efficient way of finding the original key - you'd have to iterate over the dictionary :(

---

¹ I was aware that you could iterate over the dictionary entries and find the appropriate entry that way, but I can see no reason why you'd ever want to do so when you've got a perfectly good indexer which is O(1) instead of O(N).

Solution 2

As Dictionary<TKey, TValue> implements IEnumerable<KeyValuePair<TKey, TValue>>, you could use linq:

var pair = _dictionary.SingleOrDefault(p => p.Key == myKey);

Solution 3

We can't get to "IPHone" this way:

var dict = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase)
               {
                   { "IPHone", "TCP/IP honing tools" }
               };

Console.WriteLine(dict["iPhone"]); // "TCP/IP honing tools"
Console.WriteLine( ??? ); // "IPHone"

There's seems to be no O(1) solution with the current API, but looping through all entries works:

var keyValue = dict.First(p => dict.Comparer.Equals(p.Key, "iPhone"));

Console.WriteLine(keyValue.Key); // "IPHone"
Console.WriteLine(keyValue.Value); // "TCP/IP honing tools"

Or as an extension for the lazy:

[Pure]
public static KeyValuePair<TKey, TValue> GetEntry<TKey, TValue>(this Dictionary<TKey, TValue> dictionary, TKey key)
{
    var comparer = dictionary.Comparer;
    return dictionary.FirstOrDefault(p => comparer.Equals(p.Key, key));
}

Comments

[Jon Skeet]

Well yes, you could do that... but it's staggeringly inefficient compared with just fetching the key. (It also assumes that == has been overloaded for the key type, and that the dictionary is using that comparison.) Why would you ever want to do that rather than creating a new KVP directly?

[user200783]

My line of thinking was: "It seems I can't get a reference to the actual KeyValuePair object within the dictionary. I wonder why not?". That KeyValuePair is a value type clearly answers that. Thanks Jon

[Oliver Benning]

its wrong!!! very bad code avoid it!!! consider a dictionary of 9999999 items, u can go to your item directly and create this pair, or go through lots of items and get it... think about it..

[supercat]

Unfortunately, the above only works if the key parameter and the key stored in the dictionary are semantically equal. If one has a Dictionary<string,string> called dic constructed with StringComparer.InvariantCultureIgnoreCase that contains {"Jon","Skeet"}, then dic.GetEntry("JON") would return {"JON","Skeet"}. The only way I know to make a dictionary return the key on a lookup is to have the key be part of the value. Seems silly, but I know of no alternative.

[Adarsha]

Using Linq may not be necessarily bad, and does not mandates a itearion through all the elements. It may be using internal hastable to get the single KeyValuePair. If you are using a foreach to iterate, then thats bad coding I guess!

[Boris B.]

@Adarsha: No, SingleOrDefault is an extension method for IEnumerable, which means that it treats the Dictionary as an Enumerable (i.e. relies only on IEnumerable.GetEnumerator()), which is iterative by nature.

[Levitikon]

This is perfect when trying to compare same instances of KeyValuePairs without having to create a custom comparer, especially for under the hood work.

[supercat]

@eglasius: I really wish Sun and Microsoft had recognized that in many cases it's useful to have the exact key that is stored in a dictionary or set. The lack of such ability is especially detrimental to Java's WeakHashMap [I can see stronger usage cases for a WeakIdentityHashMap, or a WeakReflexiveSet [that maps keys to themselves] than for a non-identity-based WeakHashSet that can't readily read back stored key values.

[supercat]

@JonSkeet: Glad we agree about something. Referring back to your post, do you think it might be worth making mention of the point in my earlier comment? The fact that Dictionary accepts an IEqalityComparer implies that it is not at all implausible that the key being searched for might be significantly different from the one in the dictionary.

[Jon Skeet]

@supercat: Yup, will do.

[Jim Balter]

"it's staggeringly inefficient compared with just fetching the key" -- there is no way to fetch a key from a Dictionary other than by enumerating it.

[Jim Balter]

" does not mandates a itearion through all the elements" -- Single[OrDefault] certainly does. "It may be using internal hastable to get the single KeyValuePair" -- well of course Dictionary has an internal hash table, but that's irrelevant. A hash table doesn't work with an arbitrary comparison predicate.

[ToolmakerSteve]

Note: AsEnumerable is indeed one way to "loop through all the entries", but does not solve the original question, which is whether there is an API that will let you get at the exact key, without going through all the entries. [To which the answer is still "No".]

[Dylan]

This was useful to me as I needed to add all of a dictionaries values to a list of KeyValuePairs. as I still needed the keys in the list, but they could be duplicated. And I just went exampleList.AddRange(dictionary) and it worked great.