using System;

using System.Linq;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        Dictionary<string, string> delta = new Dictionary<string, string>();

        delta.Add("A", "One");

        delta.Add("B", "Two");

        delta.Add("C", "Three");

        // Approach 1 - Dictionary Values are not unique

        var lookup = delta.ToLookup(c=>c.Value, c=>c.Key);

        Console.WriteLine( ((IEnumerable<string>)lookup["One"]).First());

        // Appraoch 2 - Dictionary Values are unique, construct transformed dictionary.

        var dict = delta.ToDictionary(kp => kp.Value, kp => kp.Key);

        dict["One"].Dump();

    }

}