HashSet with Index Access

Question

I would need a data structure that

1. Allows me to add/item to it
2. Do not allow duplication
3. access the collection via index

I am t

Answer 1

How about a collection derived from KeyedCollection? This represents a collection of items where each key is derived from the item itself. By default it does not allow you to add duplicates (i.e. items with the same key). It allows lookup by key or index.

internal class Program
{
    private static void Main(string[] args)
    {
        TestItemCollection items = new TestItemCollection();
        items.Add(new TestItem("a"));
        items.Add(new TestItem("a")); // throws ArgumentException -- duplicate key

        TestItem a = items["a"];
        a = items[0];
    }

    private sealed class TestItem
    {
        public TestItem(string value)
        {
            this.Value = value;
        }

        public string Value { get; private set; }
    }

    private sealed class TestItemCollection : KeyedCollection
    {
        public TestItemCollection()
        {
        }

        protected override string GetKeyForItem(TestItem item)
        {
            return item.Value;
        }
    }
}