Get dictionary key by value

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.

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.

I was in a situation where LINQ binding was not available and had to expand lambda explicitly. It resulted in a simple function:

public static T KeyByValue<T, W>(this Dictionary<T, W> dict, W val)
{
    T key = default;
    foreach (KeyValuePair<T, W> pair in dict)
    {
        if (EqualityComparer<W>.Default.Equals(pair.Value, val))
        {
            key = pair.Key;
            break;
        }
    }
    return key;
}

Call it like follows:

public static void Main()
{
    Dictionary<string, string> dict = new Dictionary<string, string>()
    {
        {"1", "one"},
        {"2", "two"},
        {"3", "three"}
    };

    string key = KeyByValue(dict, "two");
    Console.WriteLine("Key: " + key);
}

It works on .NET 2.0 and in other limited environments.

public static string GetKeyFromValue(string valueVar)
{
    foreach (string keyVar in dictionaryVar.Keys)
    {
        if (dictionaryVar[keyVar] == valueVar)
        {
            return keyVar;
        }
    }
    return null;
}

Other people may have more efficient answers, but I find this personally more intuitive and it works in my case.