Get all keys in Dictionary containing value x

Question

I have this:

Dictionary<integer, string> dict = new Dictionary<integer, string>();

I want to select all the items in the dictionary that contain the value abc.

Is there an inbuilt function that lets me do this easily?

Solution

Well it's reasonably simple with LINQ:

var matches = dict.Where(pair => pair.Value == "abc")
                  .Select(pair => pair.Key);

Note that this won't be even slightly efficient - it's an O(N) operation, as it needs to check every entry.

If you need to do this frequently, you may want to consider using another data structure - Dictionary<,> is specifically designed for fast lookups by key.

Comments

[jmrnet]

You could if you want to get the keys that have a value that contains "abc" you could also: var matches = dict.Where(pair => pair.Value.Contains("abc")) .Select(pair => pair.Key);

[Jon Skeet]

@User: Dictionary<,> is just shorthand for Dictionary<TKey, TValue>. How big is your dictionary? How many values do you need to search for? Once per second isn't very often, if it's a reasonably small dictionary and you're only looking for a single value...

[Elmo]

The dictionary's capacity is 10 keys and 4 values ("Left","Right","Front","Back"). And sometimes the function runs about 10 times per second.

[Casperah]

For such a small collection you shouldn't worry, However, if you don't use the special indexing feature of Dictionary you might be better off with a List<Tuple<int, string>>.

[stephanmg]

@JonSkeet: Which data structure would you recommend then?

[Jon Skeet]

@stephanmg: I'd need to know more about the precise requirements. If you have such requirements, I suggest you ask about that in a new question, with details.

[stephanmg]

@JonSkeet: You are right. I'd assume the OP wanted a "bidirectionally" fast access. In this case one could use a second dictionary in which key/val pairs are inverted.