Preventing Duplicate Selections with C# and jQuery Validation

by Brian Koser

A few days ago, while working on a web page, I got to write my favorite type of code: validation.

jQuery validation

Ha ha. Actually I’d rather floss with a brick than write validation.

Anyway, I was writing validation for an ASP.NET/C# registration page. The page has three dropdown lists, each with the same list of courses. Students can register for up to three courses by choosing a course in each dropdown list. The validation I was writing needed to prevent a student from registering for a course multiple times. It seemed like there should be a simple solution, but it wasn’t jumping out at me, so I started to pseudocode a naive solution.

Okay, not very elegant. And not easily scalable if when I’m asked to add a fourth dropdown. I started coding it out a little more in the hopes of seeing a pattern.

Then I had a “eureka” moment: I could put the values in an array, get the count of unique items, and check if the number of values is equal to the number of unique values.

This works because I don’t care how many duplicates there are, or which values are duplicated. I just want to know if duplicates exist.

However, it’s not quite complete. If a student registers for only one course, say “History 101”, dropdown list 1 will have the value “HI 101” and dropdowns 2 and 3 will be empty strings. Since "" == "", I’ll need to allow multiple empty strings.

Boom. And in jQuery:

I still don’t like writing validation, but…it has its interesting moments. ∎