/// <summary>
/// گرفتن کالا و تعداد
/// </summary>
/// <param name="stuff"></param>
/// <param name="count"></param>
public InvoiceItem(Stuff stuff, int count)
{
_stuff = stuff;
_count = count;
}
public void AddItemToInvoice(Stuff stuff, int count)
{
InvoiceItem temp = new InvoiceItem(stuff, count);
_items.Add(temp);
}