The outcomes of my repeated vending machine kata adventures

describe_VendingMachine.cs

using System;
using System.Collections.Generic;
using NSpec;

namespace VendingMachineKata
{
    class describe_VendingMachine : nspec
    {
        VendingMachine machine;
        string firstSnickersVend;
        string secondSnickersVend;

        void an_empty_machine()
        {
            before = () => machine = new VendingMachine();

            specify = () => machine.Vend("A1").Is("not in stock");

            context["doritos are stocked in A1, and 2 snickers in A2"] = () =>
            {
                before = () =>
                {
                    machine.Stock("doritos", "A1");
                    machine.Stock("snickers", "A2");
                    machine.Stock("snickers", "A2");
                };

                specify = () => machine.Vend("A1").Is("doritos");

                context["snickers are vended"] = () =>
                {
                    before = () => firstSnickersVend = machine.Vend("A2");

                    specify = () => firstSnickersVend.Is("snickers");

                    context["another snickers is vended"] = () =>
                    {
                        before = () => secondSnickersVend = machine.Vend("A2");

                        specify = () => secondSnickersVend.Is("snickers");

                        specify = () => machine.Vend("A2").Is("not in stock");
                    };
                };
            };
        }
    }

    class VendingMachine
    {
        public VendingMachine()
        {
            stock = new StockDictionary<string, Item>();
        }

        public void Stock(string name, string slot)
        {
            stock.AddStock(slot, () => new Item(name));
        }

        public string Vend(string slot)
        {
            return stock.ContainsKey(slot) ? stock.Get(slot).Name : "not in stock";
        }

        StockDictionary<string, Item> stock;
    }

    class StockDictionary<T, U> : Dictionary<T, U> where U : IQuantity
    {
        public void AddStock(T key, Func<U> value)
        {
            if (!ContainsKey(key))
                Add(key, value());
            else
                this[key].Quantity++;
        }

        public U Get(T slot)
        {
            var item = this[slot];

            if (item.Quantity == 1)
                Remove(slot);
            else
                item.Quantity--;

            return item;
        }
    }

    class Item : IQuantity
    {
        public Item(string name)
        {
            Name = name;

            Quantity = 1;
        }

        public string Name { get; set; }

        public int Quantity { get; set; }
    }

    interface IQuantity
    {
        int Quantity { get; set; }
    }
}