C# HashSet but Want to Force Exception When Adding Duplicates

Question

I want to use a HashSet but worry that it "silently" absorbs duplicates (by returning true/false) but not by throwing an Exception. I'm interested in an equivalent data structure that does throw an exception (like a Dictionary would).

What's the best approach for this?

Answer 1

You can just define an extension method on HashSet<T> (and trust it a bit more not to have to).

public static class Extensions {
    public static void AddWithThrow<T>(this HashSet<T> hashSet, T value) {
        if (hashSet.Add(value) == false) {
            throw new ArgumentException("An item with the same value has already been added");
        }
    }
}

and if for some reason you had an ICollection<T> accepting method that wants throwing semantics on Add, maybe this:

public class ThrowingHashSet<T> : HashSet<T>, ICollection<T> {

    // uncomment if you can live with the confusion
    // of HashSet<T> variables and HashSet<T> accepting methods
    // calling the existing HashSet<T>.Add non-throwing method instead

    // public new void Add(T value) => AddWithThrow(value);

    private void AddWithThrow(T value) {
        if (base.Add(value) == false) {
            throw new ArgumentException("An item with the same value has already been added");
        }
    }
    void ICollection<T>.Add(T item) => this.AddWithThrow(item);
}