interface.cs

public interface ISet<ItemT>
{
	void Add(ItemT item);
	void Remove(ItemT item);
	bool Contains(ItemT item);
	IEnumerable<ItemT> Items { get; }
	int Count { get; }
}

interface_alternative.cs

public sealed class ISet<ItemT>
{
	public Action<ItemT> Add = delegate { };
	public Action<ItemT> Remove = delegate { };
	public Func<ItemT, bool> Contains = delegate { return false; };
	public Func<IEnumerable<ItemT>> Items = () => Enumerable.Empty<ItemT>();
	public Func<int> Count = () => 0;
}