It seems like a pretty straightforward thing to add, but I just want to be sure .NET doesn't already provide one and save me from adding unnecessary code:

I need a lookup table (like a Dictionary) that instead using a key/value pair, uses the first key to find the second key, and vice versa. For example -

theList.Add("foo", "bar");
x = theList["foo"]; // Returns "bar"
y = theList["bar"]; // Returns "foo"

Thanks in advance!

  • 1
    What would this collection do when someone performs: theList.Add("foo", "bar"); followed by theList.Add("bar", "baz"); ?
    – rwong
    Commented Aug 2, 2015 at 7:39
  • 1
    This is a bidirectional map, usually called BidiMap or BiMap (or, since .NET calls maps dictionaries, in .NET, it would probably be called BidiDictionary, BiDictionary, or, if you want to be extra clever Bictionary). AFAIK, there is no such thing in the BCL, but you might find it in one of the other collections packages, e.g. C5 or PowerCollections. There are also about 20 questions about this in StackOverflow. Commented Aug 2, 2015 at 9:41
  • rwong - I would expect it to throw an exception in that case. Check my answer below -- this is what I was looking to achieve, I just wanted to know if it already had an implementation. Jörg - Thank you for this! I didn't know the name so I had no idea what to search for. Good to know. Commented Aug 2, 2015 at 9:45

2 Answers 2


There is no such datatype, probably because this is a very special requirement which can be easily solved by utilizing a dictionary and adding simply both pairs

 theDictionary.Add("foo", "bar");
 theDictionary.Add("bar", "foo");

Obviously, you can put this into a generic function like

void MyDictAdd(Dictionary<T,T> dict, T key1, T key2)
  • Also, lookups are done with dict[key] which you forgot to mention! :) So if you have a dictionary as such: var dict = new Dictionary<string, string> { { "somekey", "somevalue" } }; then dict["somekey"] will return "somevalue".
    – mausworks
    Commented Aug 2, 2015 at 11:24
  • Genius solution. But to implement Count and other functionality, logic is needed
    – Ozkan
    Commented Oct 11, 2018 at 11:58
  • This solution has one main drawback: key and value have to be the same type.
    – Zoman
    Commented Dec 31, 2018 at 19:36
  • @Zoman: that is not a "drawback of this solution", it is what the question necessarily implies. Or, if one needs mixed types, one can still use object as common type.
    – Doc Brown
    Commented Jan 1, 2019 at 1:28

Another possibility would be to create a method which would search both keys and values. Also this would and more complexity to the mix O(n) for the search. But would reduce memory usage.

string GetValue(string search, Dictionary<string, string> dictionary) {
    foreach(var pair in dictionary)
        if (pair.Key == search) {
            return pair.Value;
        else if (pair.Value == search) {
            return pair.Key;
    return null;

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