How to centrally maintain a mathematical formula in C# (web) so it can be changed if needed?

Question

We have an application that has a LOT of mathematical checks on the page and according to it, the user is given a traffic light (Red, green, yellow).

Green = He may continue
Red = Dont let him continue
Yellow = Allow to continue but warn

These formulas operate on the various text-fields on the page. So, for example, if textbox1 has "10" and texbox2 has "30"... The formula might be:
T1 * T2 > 600 ? "GREEN" : "RED"

My question is:
Is it possible to somehow centralize these formulas?

Why do I need it?
Right now, if there is any change in a formula, we have to replicate the change at server-side as well (violation of DRY, difficult to maintain code)

One option could be to
- store the (simple) formula as text with placeholders in a config(?)
- replace the placeholders with values in javascript as well as server-side code
- use eval() for computation in JS
- use tricks outlined here for C#

In this approach issue could be different interpretations of same mathematical string in JS and C#.

Am i making sense or should this question be reported?!! :P

Answer 1

Depending on your application's requirements, it may be acceptable to just do all the validation on the server. Particularly if you have few users or most of them are on a reasonably fast intranet, you can "waste" some network calls to save yourself a maintenance headache.

If the user wants feedback between every field entry (or every few entries, or every few seconds), you could use an AJAX call to ask the server for validation without a full page refresh.

This will, of course result in more requests than doing the validation entirely on the client, and if many of your users have bad network connections there could be latency in giving them the feedback. My guess is the total bandwidth usage is about the same. You use some for every validation round-trip, but those are small. It may be outweighed by all that validation JS that you're not going to send to clients.

The main benefit is the maintenance and FUD that you'd otherwise have keeping the client and server validation in sync. There's also the time savings in never having to write the validation javascript.

In any case, it may be worth taking a step back and asking what your requirements are.