Get dictionary key by value

Question

How do I get a Dictionary key by value in C#?

Dictionary<string, string> types = new Dictionary<string, string>() {     {"1", "one"},     {"2", "two"},     {"3", "three"} }; 

I want something like this:

getByValueKey(string value); 

getByValueKey("one") must be return "1".

What is the best way do this? Maybe HashTable or SortedLists?

Answer 1

Values do not necessarily have to be unique, so you have to do a lookup. You can do something like this:

var myKey = types.FirstOrDefault(x => x.Value == "one").Key; 

If values are unique and are inserted less frequently than read, then create an inverse dictionary where values are keys and keys are values.

Answer 2

You could do that:

1. By looping through all the KeyValuePair<TKey, TValue>'s in the dictionary (which will be a sizable performance hit if you have a number of entries in the dictionary)
2. Use two dictionaries, one for value-to-key mapping and one for key-to-value mapping (which would take up twice as much space in memory).

Use Method 1 if performance is not a consideration, and use Method 2 if memory is not a consideration.

Also, all keys must be unique, but the values are not required to be unique. You may have more than one key with the specified value.