How to make a ReadOnlyCollection from a HashSet without copying the elements?

Question

I have a private HashSet<string> which is the backing field of a read-only property which should return a read-only collection such that callers cannot modify the collection. So I tried to:

public class MyClass
{
    private readonly HashSet<string> _referencedColumns;

    public ICollection<string> ReferencedColumns { 
        get { return new ReadOnlyCollection<string>(_referencedColumns); }
    }

This does not compile as ReadOnlyCollection accepts a IList<T> which is not implememted by HashSet<T>. Is there another wrapper I can use to save me from copy the items? For my purpose it is enough to just return something implementing ICollection<T> (instead of IList<T>) which is implemented by the HashSet<T>.

Answer 1

Consider exposing the property as the type IReadOnlyCollection<> instead, which will provide a read-only view of the HashSet<>. This is an efficient way of implementing this, since the property getter will not require a copy of the underlying collection.

This will not prevent someone from casting the property to a HashSet<> and modifying it. If you are concerned with that, consider return _referencedColumns.ToList() in the property getter, which will create a copy of your underlying set.