Question

A HashSet<T> can determine in O(1) whether it contains a certain item. If I override Equals() and GetHashCode() on my custom class, I can have an object A and another object A’ that are not equal by identity but for which Equals() returns true and GetHashCode() returns the same hash code.

Now, given that A is in the hash set, I want to retrieve A in O(1) given A’ (which is equal to A from the perspective of the hash set).

var a = new MyClass("A");
var a_prime = new MyClass("A");
Debug.Assert(a.Equals(a_prime));
Debug.Assert(a.GetHashCode() == a_prime.GetHashCode());

var set = new HashSet<MyClass>();
set.Add(a);
Debug.Assert(set.Contains(a_prime));

// This:    
var retrieved_a = set.Get(a_prime);

How to do this?

(Note that this has not the answer I’m looking for, and this has no answers at all.)

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Some background information: I want to use the set to intern my own objects the same way C# interns strings: equal objects need only one instance. This way I can append metadata to such an object and be sure that there is no other equal instance anywhere without that metadata.

Answer 1

There is no method on HashSet that does what you want.

You can use a Dictionary instead:

var dict = new Dictionary<MyClass, MyClass>();
dict[a] = a;
Debug.Assert(dict.ContainsKey(a_prime));
var retrieved_a = dict[a_prime];