C# Dictionary search predicate as method argument

Question

I am trying to create a function whereby I can pass in a functor/predicate that can slot into a dictionary's 'Where' method.

(cardPool is the dictionary of type 'cardStats') Pseudo of what I'd like to do:

void CardStats findCard(Predicate<CardStats> pred)
{
    return cardPool.Where(pred);
}

This code obviously wont work but is simply a rough example of the functionality I am looking for. I have had no problems setting this up for lists, but for a Dictionary, its really got me stumped.

Any help would be great, thanks!

Edit: Ahh sorry I should have mentioned more: Cardstats is the value, the key is of type int. I'd like to sift through the values (cardStats) and test their properties such as ID(int) or name(string).

Dictionaries have two generic type parameters (the key type and the value type). Your dictionary can't just have one type. — JLRishe, Dec 23 '14 at 21:15
Well is `CardStats` the value type or the key type? It sounds like you may just want to use either the `Values` or `Keys` property, then use `Where` as normal... — Jon Skeet, Dec 23 '14 at 21:15

JLRishe · Accepted Answer · 2014-12-23T21:50:14.107

7

Dictionary<TKey, TValue> implements IEnumerable<KeyValuePair<TKey, TValue>>, so its Where extension method takes a predicate of type Func<KeyValuePair<TKey, TValue>, bool>.

You could implement your method like this:

void CardStats findCard(Func<int, CardStats, bool> pred)
{
    return cardPool.Where(kv => pred(kv.Key, kv.Value))
                   .Select(kv => kv.Value)
                   .FirstOrDefault();
}

And use it like this:

CardStats stats = myCards.findCard((id, stats) => id == 7);

or

CardStats stats = myCards.findCard((id, stats) => stats.Name == "Ace of Clubs");

Note that using Where on a dictionary doesn't take advantage of the dictionary's quick lookup features and basically treats it as a linear collection of key-value pairs.

One more comment: I would suggest providing a method that returns an IEnumerable of found cards if there are several. Or you could provide one that does that, and one that just returns the first match:

void IEnumerable<CardStats> findCards(Func<int, CardStats, bool> pred)
{
    return cardPool.Where(kv => pred(kv.Key, kv.Value))
                   .Select(kv => kv.Value);
}

void CardStats findCard(Func<int, CardStats, bool> pred)
{
    return findCards(pred).FirstOrDefault();
}

edited Dec 23 '14 at 21:50

answered Dec 23 '14 at 21:22

JLRishe

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Ahh sorry I should have mentioned more: Cardstats is the value, the key is of type int. I'd like to sift through the values (cardStats) and test their properties such as ID(int) or name(string). – Dahlvash Dec 23 '14 at 21:25
"key-value" is important to look for. Performance you need to consider in large dictionary. – Chaturvedi Dewashish Dec 23 '14 at 21:27
You can also do cardPool.Where(pred).ToList() to get all the values matching the pred – Chaturvedi Dewashish Dec 23 '14 at 21:29
@ChaturvediDewashish You can't directly pass a `Func` to the `Where()` method because it expects a `Func, bool>`. I chose to use the former in the method signature because it allows writing lambdas like `(id, stat) => id == 5` instead of `kv => kv.Key == 5`, which is less clear in its intent. – JLRishe Dec 23 '14 at 21:34
@JLRishe Thanks. Nice catch. Actually I had wrapped it as Expression>. It helped me build complex predicates. http://stackoverflow.com/questions/11490893/how-does-predicatebuilder-work – Chaturvedi Dewashish Dec 23 '14 at 21:38
Thank you, JLRishe! Just gave it a whirl and it does exactly what I was looking for. – Dahlvash Dec 23 '14 at 21:49

score 1 · Answer 2 · answered Apr 19 '20 at 22:55

1

I would use FirstOrDefault as the first statement because it will stop as soon it finds a matching element. another thing is that I will consider using something else than a dictionary - because when using it this way is abuse if its indexed purpose.

anyway, this is the code I will use:

public CardStats Find(Func<CardStats, bool> predicate)
{
    KeyValuePair<int, Roster> kvCard = cardPool.FirstOrDefault(kvp => predicate(kvp.Value));
    if (kvCard.Equals(default(KeyValuePair<int, Roster>)))
        return null;
    return kvCard.Value;
}

answered Apr 19 '20 at 22:55

EladTal

2,167
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LINQ uses lazy evaluation, so it will not go through the remainder of the collection after the first item is found if you use `.Where().Select().FirstOrDefault()`. `.FirstOrDefault(pred)` is a little more concise, but then you need a null check as you have here. You can use the `?.` operator to simplify your code a bit, but my approach allows implementing select-multi and select-single helpers without duplicating code. – JLRishe Sep 03 '22 at 10:42

C# Dictionary search predicate as method argument

2 Answers2