Two-way key value collection [duplicate]

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Is there a collection in C# working like dictionary with multiple keys allowing bidirectional access? For example:

Collection<int, string> a = new Collection<int, string>();
a.Add(1, "a");
a.Add(1, "b");
a.Add(2, "a");
a.Get(1);//returns ["a", "b"]
a.InverseGet("a"); //returns [1, 2]


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.Where()

var collection = new List<KeyValuePair<int, string>>();

2 Answers
2

Using Tuple List<Tuple<int,string>> to allow a collection with the same key and LINQ to make a query Where to your collection

List<Tuple<int,string>>

LINQ

For example,

List<Tuple<int,string>> list = new List<Tuple<int,string>>();
list.Add(Tuple.Create(23, "Foo"));
list.Add(Tuple.Create(23, "Bar"));
list.Add(Tuple.Create(25, "Bar"));

var keys = list.Where(x=> x.Item1 == 23).Select(x=> x.Item2); // FOO, BAR
var values = list.Where(x=> x.Item2 == "Bar").Select(x=> x.Item1); ; // 23, 25


Here is a solution that gives you the performance of a dictionary lookup in both directions. I assume that you want to ignore duplicate tuples and the default comparer is sufficient. Implementing the other operations like Remove is left as an exercise for the reader

public class TwoWayCollection<A, B>

private Dictionary<A, HashSet<B>> byADictionary = new Dictionary<A, HashSet<B>>();
private Dictionary<B, HashSet<A>> byBDictionary = new Dictionary<B, HashSet<A>>();

public IEnumerable<B> Get(A a)

return byADictionary[a];


public IEnumerable<A> InverseGet(B b)

return byBDictionary[b];


public void Add(A a, B b)

if (!byADictionary.ContainsKey(a))

byADictionary[a] = new HashSet<B>();

byADictionary[a].Add(b);
if (!byBDictionary.ContainsKey(b))

byBDictionary[b] = new HashSet<A>();

byBDictionary[b].Add(a);




Then to use it is literally your proposed code. Both Get and InverseGet approach O(1) as per dictionary

TwoWayCollection<int, string> a = new TwoWayCollection<int, string>();
a.Add(1, "a");
a.Add(1, "b");
a.Add(2, "a");
a.Get(1); //returns ["a", "b"]
a.InverseGet("a"); //returns [1, 2]