How to retrieve actual item from HashSet<T>?

Question

I've read this question about why it is not possible, but haven't found a solution to the problem.

I would like to retrieve an item from a .NET HashSet<T>. I'm looking for a method that would have this signature:

/// <summary> /// Determines if this set contains an item equal to <paramref name="item"/>,  /// according to the comparison mechanism that was used when the set was created.  /// The set is not changed. If the set does contain an item equal to  /// <paramref name="item"/>, then the item from the set is returned. /// </summary> bool TryGetItem<T>(T item, out T foundItem); 

Searching the set for an item with such a method would be O(1). The only way to retrieve an item from a HashSet<T> is to enumerate all items which is O(n).

I haven't find any workaround to this problem other then making my own HashSet<T> or use a Dictionary<K, V>. Any other idea?

Note:
I don't want to check if the HashSet<T> contains the item. I want to get the reference to the item that is stored in the HashSet<T> because I need to update it (without replacing it by another instance). The item I would pass to the TryGetItem would be equal (according to the comparison mechanism that I've passed to the constructor) but it would not be the same reference.

Answer 1

This is actually a huge omission in the set of collections. You would need either a Dictionary of keys only or a HashSet that allows for the retrieval of object references. So many people have asked for it, why it doesn't get fixed is beyond me.

Without third-party libraries the best workaround is to use Dictionary<T, T> with keys identical to values, since Dictionary stores its entries as a hash table. Performance-wise it is the same as the HashSet, but it wastes memory of course (size of a pointer per entry).

Dictionary<T, T> myHashedCollection; ... if(myHashedCollection.ContainsKey[item])     item = myHashedCollection[item]; //replace duplicate else     myHashedCollection.Add(item, item); //add previously unknown item ... //work with unique item 

Answer 2

What you're asking for was added to .NET Core a year ago, and was recently added to .NET 4.7.2:

In .NET Framework 4.7.2 we have added a few APIs to the standard Collection types that will enable new functionality as follows.
- ‘TryGetValue‘ is added to SortedSet and HashSet to match the Try pattern used in other collection types.

The signature is as follows (found in .NET 4.7.2 and above):

    //     // Summary:     //     Searches the set for a given value and returns the equal value it finds, if any.     //     // Parameters:     //   equalValue:     //     The value to search for.     //     //   actualValue:     //     The value from the set that the search found, or the default value of T when     //     the search yielded no match.     //     // Returns:     //     A value indicating whether the search was successful.     public bool TryGetValue(T equalValue, out T actualValue); 

P.S.: In case you're interested, there is related function they're adding in the future - HashSet.GetOrAdd(T).