How to affect something to a variable each day C# asp.net

Question

I have a database with some prices in it. For example I have milk = 1€. I want the price not to be modify for more or less than 10%. So in this case you can set the price only between 0.90 or 1.10. It's easy to do that, however, when the price is modified, the user can modify it again right after. (if he sets it to 0.9, then he can set it between 0.81 and 0.99 and so on...)So I want to affect in a variable the price for example each day at 9 a.m, so in one day it can't change for more or less than 10%.

DataClassesDataContext db = new DataClassesDataContext();

List<ARTICLE> article = db.ARTICLE.ToList();

for (int i = 0; i < article.Count; i++)
{
    switch (article[i].Name)
    {
        case "Milk":
            if (decimal.Parse(Milk.Text, CultureInfo.InvariantCulture) <= 
                (article[i].Price/ 10) * 11 && decimal.Parse(Milk.Text,
                 CultureInfo.InvariantCulture) >= (article[i].Price/ 10) * 9)
            {
                if (decimal.Parse(Milk.Text, CultureInfo.InvariantCulture) != article[i].Price)
                {
                    article[i].Price= decimal.Parse(Milk.Text, CultureInfo.InvariantCulture);
                }
            }
            break;
        case "Other thing":
            //... etc
     }
}

Answer 1

So you want to:

• Save the start price for each day
• Allow the user to change the price for that day, within a threshold of N%
• Allow the user to change the price multiple times a day (to fix corrections, or to increase or decrease the price as long as the total difference is < N%)

Then upon changing the daily price, check that it doesn't vary from the base price by more than N%.

To solve this, you'd use three columns: DailyPrice, CurrentPrice, PriceModified. Then upon editing a price:

if (record.PriceModified.Date != DateTime.Now.Date)
{
    // First edit for today. 
    if (PriceWithinChangeThreshold(formData.NewPrice, record.CurrentPrice))
    {       
        // Save the old price as the new start price for today
        record.DailyPrice = record.CurrentPrice;
        record.CurrentPrice = formData.NewPrice;
        record.PriceModified = DateTime.Now;
    }
}
else
{
    // This price has already been edited today.
    if (PriceWithinChangeThreshold(formData.NewPrice, record.DailyPrice))
    {
        // But it's within the threshold for today
        record.CurrentPrice = formData.NewPrice;
        record.PriceModified = DateTime.Now;
    }   
}

But as discussed in comments, saving prices as mere decimal columns is generally a bad idea. Think about (historical) reporting, invoicing, taxes, currency and whatnot: a "product price" is way more than a number. So I'd suggest storing your prices in a different manner altogether. See mjwlls' answer for that.

Answer 2

One possible solution for this would be to add a PriceHistory table.

Then whenever you save a price, you check whether the new price is valid and (if it is) add a record to the PriceHistory table.

The check would be something like:

SELECT TOP 1 MostRecentPrice
FROM PriceHistory
WHERE Item = 'Milk' and PriceDateTime < 'Today's Date (with midnight as the time)'
ORDER BY PriceDateTime DESC

Then verifying that the new price you are saving is within 10% of MostRecentPrice. Also, make sure you handle the scenario where there is no recent price (i.e. this is the first time you have saved the price for an item).

Another advantage of this is that not only does it solve your current problem, it gives you valuable information over time. "What was the price of milk 3 weeks ago?" That kind of thing.

Plus, you can alter the rules of the validation reasonably simply - for example you could say 'no price changes of 10% per day or 20% per week' - which you can't do as easily if you don't store the full history.

Answer 3

You can save last modification date time and can validate it upon every change.

Answer 4

You could introduce a lastModified-member in all your Articles. Thus you know if you may or may not modify the current product:

class Article
{
    DateTime LastModified { get; set; }
}

...

foreach(var article in articles)
{
    var timespan = DateTime.Now - article.LastModified;
    if(timeSpan.TotalDays >= 1) ...
}

Answer 5

Create a column like "LastModified" as DateTime in your table. And enable the button for modification after checking if the day and month of "LastModified" is not the same as today's date.

It can't be done in the application itself or you would need something that runs on a server that you can query with a webservice or what ever.

If you can't modify the table maybe creating a local file with a last modified date but then the user can modify it so be careful.