Application Services as Open Closed...

something.cs

public class Command {}
public class GetStockCommand("name", 10M) : Command {
  //set properties
  DateAdded = DateTime.now;
}

public interface IHandle<Command, TResult>
{
  TResult Handle(Command command);
}

public class GetStockHandler : IHandle<GetStockCommand, StockItem>
{
  public StockItem Handle(GetStockCommand cmd) {
    //load from hipster database
  }
}

public class PriceReducingGetStockHandler : IHandle<GetStockCommand, StockItem>
{
  public PriceReducingGetStockHandler(GetStockHandler baseHandler)
  {_baseHandler = baseHandler;}
  
  public StockItem Handle(GetStockCommand cmd) {
    var si = _baseHandler.Handle(cmd);
    si = si.ReducePriceFor(DateTime.Now);
    return si;
  }
}