[c#] Get index of a key/value pair in a C# dictionary based on the value

I would like to know if some property or method exists that gets the index of a specific value.

I found that dictionaries have the Contains() method which returns true if the value passed in exists, so this method almost implements what I need.

I know that I can loop through all the value pairs and check the condition, but I ask because maybe there's an optimized way of doing this.

Consider using System.Collections.Specialized.OrderedDictionary, though it is not generic, or implement your own (example).

OrderedDictionary does not support IndexOf, but it's easy to implement:

public static class OrderedDictionaryExtensions
{
    public static int IndexOf(this OrderedDictionary dictionary, object value)
    {
        for(int i = 0; i < dictionary.Count; ++i)
        {
            if(dictionary[i] == value) return i;
        }
        return -1;
    }
}

You can use LINQ to help you with this.

Dictionary<int, string> dict = new Dictionary<int, string>();
dict.Add(1, "hi");
dict.Add(2, "NotHi");
dict.Add(3, "Bah");

var item = (from d in dict
            where d.Value == "hi"
            select d.Key).FirstOrDefault();

Console.WriteLine(item); //Prints 1

In your comment to max's answer, you say that what you really wanted to get is the key in, and not the index of, the KeyValuePair that contains a certain value. You could edit your question to make it more clear.

It is worth pointing out (EricM has touched upon this in his answer) that a value might appear more than once in the dictionary, in which case one would have to think which key he would like to get: e.g. the first that comes up, the last, all of them?

If you are sure that each key has a unique value, you could have another dictionary, with the values from the first acting as keys and the previous keys acting as values. Otherwise, this second dictionary idea (suggested by Jon Skeet) will not work, as you would again have to think which of all the possible keys to use as value in the new dictionary.

If you were asking about the index, though, EricM's answer would be OK. Then you could get the KeyValuePair in question by using:

yourDictionary.ElementAt(theIndexYouFound);

provided that you do not add/remove things in yourDictionary.

PS: I know it's been almost 7 years now, but what the heck. I thought it best to formulate my answer as addressing the OP, but of course by now one can say it is an answer for just about anyone else but the OP. Fully aware of that, thank you.

    You can find index by key/values in dictionary

Dictionary<string, string> myDictionary = new Dictionary<string, string>();
myDictionary.Add("a", "x");
myDictionary.Add("b", "y");
int i = Array.IndexOf(myDictionary.Keys.ToArray(), "a");
int j = Array.IndexOf(myDictionary.Values.ToArray(), "y");

If searching for a value, you will have to loop through all the data. But to minimize code involved, you can use LINQ.

Example:

Given Dictionary defined as following:

Dictionary<Int32, String> dict;

You can use following code :

// Search for all keys with given value
Int32[] keys = dict.Where(kvp => kvp.Value.Equals("SomeValue")).Select(kvp => kvp.Key).ToArray();
        
// Search for first key with given value
Int32 key = dict.First(kvp => kvp.Value.Equals("SomeValue")).Key;

no , there is nothing similar IndexOf for Dictionary although you can make use of ContainsKey method to get whether a key belongs to dictionary or not

Let's say you have a Dictionary called fooDictionary

fooDictionary.Values.ToList().IndexOf(someValue);

Values.ToList() converts your dictionary values into a List of someValue objects.

IndexOf(someValue) searches your new List looking for the someValue object in question and returns the Index which would match the index of the Key/Value pair in the dictionary.

This method does not care about the dictionary keys, it simply returns the index of the value that you are looking for.

This does not however account for the issue that there may be several matching "someValue" objects.