Product.cs

public class CatalogProduct
{
    public CatalogProduct()
    {

    }

    public CatalogProduct(string sku, string name, string description, decimal price, decimal cost)
    {
        Sku = sku;
        Name = name;
        Description = description;
        Price = price;
        Cost = cost;
    }

    public string Sku { get; private set; }
    public string Name { get; set; }
    public string Description { get; set; }
    public decimal Price { get; private set; }
    public decimal Cost { get; private set; }
    public int QuantityOnHand { get; private set; }
    public bool ForSale { get; private set; }
    public bool FreeShipping { get; private set; }

    public void SetAvailable()
    {
        if (CanSetAvailable())
        {
            ForSale = true;
        }
    }

    public bool CanSetAvailable()
    {
        return ForSale == false && QuantityOnHand > 0;
    }

    public string ValidationError()
    {
        return CanSetAvailable()
            ? null
            : "Product is must be unavailable and Quantity greater than 0";
    }

    public bool CanSetUnavailable()
    {
        return ForSale;
    }

    public void InventoryAdjustment(int adjustment)
    {
        QuantityOnHand += adjustment;
        if (QuantityOnHand <= 0)
        {
            UnavailableForSale();
        }
    }

    public void IncreasePrice(decimal newPrice)
    {
        if (newPrice < Price)
        {
            throw new InvalidOperationException("New price must be greater than current price.");
        }

        Price = newPrice;
    }

    public void DecreasePrice(decimal newPrice)
    {
        if (newPrice > Price)
        {
            throw new InvalidOperationException("New price must be less than current price.");
        }

        Price = newPrice;

        if (Price <= 0)
        {
            UnavailableForSale();
        }
    }

    public void UnavailableForSale()
    {
        ForSale = false;
    }
}