gistfile1.cs

    public class Inventory
    {
        public Dictionary<GarmentColor, int> Stock { get; set; }

        public Inventory()
        {
            Stock = new Dictionary<GarmentColor, int>();
        }

        public void Add(GarmentColor color, int amount)
        {
            if (Stock.ContainsKey(color))
            {
                Stock[color] = amount;
                return;
            }

            Stock.Add(color, amount);
        }

        public void Sell(GarmentColor color, int amount)
        {
            if (!Stock.ContainsKey(color))
                throw new Exception("There are no garments of color " + color + "  in stock!");

            var stockCount = Stock[color];

            if (amount > stockCount)
                throw new Exception("Insufficient stock to complete sale!");
        }

        public void TradeIn(GarmentColor oldGarment, GarmentColor newGarment)
        {
            if (!Stock.ContainsKey(newGarment) || Stock[newGarment] <= 0)
                throw new Exception("Can't trade garments, garment trading in for isn't in stock!");

            Stock[oldGarment]++;
            Stock[newGarment]--;
        }
    }

    public enum GarmentColor
    {
        Black,
        Blue
    }