gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections;

namespace ConsoleApplication1
{
	interface IItem
	{
	}

	class Key : IItem
	{
	}

	class Inventory : IEnumerable
	{
		private List<IItem> _items;

		public Inventory()
		{
			_items = new List<IItem>();
		}

		public void Add(IItem item)
		{
			if (_items.Contains(item))
				throw new InvalidOperationException();

			_items.Add(item);
			Console.WriteLine("ADDING ITEM!");
		}

		System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
		{
			throw new NotImplementedException();
		}
	}

	class Program
	{
		static void Main(string[] args)
		{
			var key = new Key();
			var inventory = new Inventory 
			{
				key,
				key,
 				new Key(),
 				new Key(),
 				new Key(),
 				new Key()
			};

			Console.ReadKey();
		}
	}
}