How do I keep historical variable value tied to certain period of time?

Question

I'm currently trying to figure out how I can keep ever-changing variable values and make it available for other classes to access the said variables.

In my example, I have the following:

public class Item
{
    public int Quantity;
    public int BuyPrice;
    public int SellPrice;
}

The value for all the variables in Item class will change every day.

Let's say I'm entering Day #2, how do I save class Item's variables value for Day #1 so that other classes would be able to refer to Day #1 value on Day #2?

Further to the my earlier question, there will be multiple Vendor classes having their own instances of Item and these Vendor classes will be visited by Agent classes.

I would like for the Agent to loop through all instances of Item sell price owned by the last Vendor it visited. Agent will hold a record of when it last visited the Vendor and will request for Item sell prices on that particular date by that particular Vendor.

Hope that make sense.

Answer 1

You can store them in an array or List<>, up to the maximum number of days of history you want to keep.

There are several ways to structure such a representation, but given how you phrased your query, what I would suggest would be to first wrap the changing cost data into its own type (because, presumably, some aspects of the item such as perhaps the name won't change every day):

struct CostData {
  public int Quantity;
  public int BuyPrice;
  public int SellPrice;
}

Then modify Item to hold a list of CostData

class Item {
  List<CostData> Cost;
};

Cost[Cost.Length - 1] could be "today's" data, and Cost[Cost.Length - 1 - n] would be the data for \$n\$ days ago; when the day rolls over you just insert the new day's cost onto the back of the list. I'd suggest you actually write methods to access the data though, rather than relying on consumers of the API remembering that convention.