Mutable structs in C#

We know what this does, right?

    struct Pricer
    {
        public double Price;
        public long Size;

        public void AddExecution(long lastSize, double lastPrice)
        {
            Price = (Price * Size + lastSize * lastPrice) / (Size + lastSize);
            Size += lastSize;
        }
    }

    class PriceData
    {
        public Pricer pricer;
    }

    class Program

    {

        static void Main(string[] args)
        {
            Pricer price = new Pricer{Price = 0.0, Size = 0};
            for (int i = 0; i < 5; ++i)
            {
                Console.WriteLine("{0} {1}", price.Price, price.Size);
                price.AddExecution(1, 2.5 * (i + 1));
            }

            for (int i = 0; i < 5; ++i)
            {
                Console.WriteLine("{0} {1}", price.Price, price.Size);
                price.Price= 2.5 * (i + 1);
            }

            PriceData priceData = new PriceData();
            priceData.pricer = price;
            for (int i = 0; i < 5; ++i)
            {
                Console.WriteLine("{0} {1}", price.Price, price.Size);
                priceData.pricer.AddExecution(1, 2.5 * (i + 1));
            }

       }

    }

Eric Lippert tells us about mutating *readonly* structs: http://blogs.msdn.com/b/ericlippert/archive/2008/05/14/mutating-readonly-structs.aspx but even non-readonly structs can get us in a mess.