Use LINQ to do a reverse Dictionary<K, V> lookup. But keep in mind that the values in your Dictionary<K, V> values may not be distinct.

Demonstration:

using System;
using System.Collections.Generic;
using System.Linq;

class ReverseDictionaryLookupDemo
{
    static void Main()
    {
        var dict = new Dictionary<int, string>();
        dict.Add(4, "Four");
        dict.Add(5, "Five");
        dict.Add(1, "One");
        dict.Add(11, "One"); // duplicate!
        dict.Add(3, "Three");
        dict.Add(2, "Two");
        dict.Add(44, "Four"); // duplicate!

        Console.WriteLine("\n== Enumerating Distinct Values ==");
        foreach (string value in dict.Values.Distinct())
        {
            string valueString =
                String.Join(", ", GetKeysFromValue(dict, value));

            Console.WriteLine("{0} => [{1}]", value, valueString);
        }
    }

    static List<Int32> GetKeysFromValue(Dictionary<int, string> dict, string value)
    {
        // Use LINQ to do a reverse dictionary lookup.
        // Returns a 'List<T>' to account for the possibility
        // of duplicate values.
        return
            (from item in dict
             where item.Value.Equals(value)
             select item.Key).ToList();
    }
}

Expected Output:

== Enumerating Distinct Values ==
Four => [4, 44]
Five => [5]
One => [1, 11]
Three => [3]
Two => [2]

Use LINQ to do a reverse Dictionary<K, V> lookup. But keep in mind that the values in your Dictionary<K, V> values may not be distinct.

Demonstration:

using System;
using System.Collections.Generic;
using System.Linq;

class ReverseDictionaryLookupDemo
{
    static void Main()
    {
        var dict = new Dictionary<int, string>();
        dict.Add(4, "Four");
        dict.Add(5, "Five");
        dict.Add(1, "One");
        dict.Add(11, "One"); // duplicate!
        dict.Add(3, "Three");
        dict.Add(2, "Two");
        dict.Add(44, "Four"); // duplicate!

        Console.WriteLine("\n== Enumerating Distinct Values ==");
        foreach (string value in dict.Values.Distinct())
        {
            string valueString =
                String.Join(", ", GetKeysFromValue(dict, value));

            Console.WriteLine("{0} => [{1}]", value, valueString);
        }
    }

    static List<int> GetKeysFromValue(Dictionary<int, string> dict, string value)
    {
        // Use LINQ to do a reverse dictionary lookup.
        // Returns a 'List<T>' to account for the possibility
        // of duplicate values.
        return
            (from item in dict
             where item.Value.Equals(value)
             select item.Key).ToList();
    }
}

Expected Output:

== Enumerating Distinct Values ==
Four => [4, 44]
Five => [5]
One => [1, 11]
Three => [3]
Two => [2]