Query upcoming birthdays

Question

I'd like to query my customers for the ones, whose birthdays are yet to come.

I've tried this query, and it - of course - has failed breathtakingly:

Addresses.Where(adr => adr.DateOfBirth != null && adr.DateOfBirth.Value >
DateTime.Now).Take(15).ToList();

Of course this can't work properly (not if you're born in the future) and I'd like to know how I can query my Nullable<DateTime> without a year?

Answer 1

You can do it in one line like this:

context.Addresses.Where(adr => adr.DateOfBirth != null).OrderBy(adr => EntityFunctions.DiffDays(DateTime.Today, EntityFunctions.AddYears(adr.DateOfBirth, EntityFunctions.DiffYears(adr.DateOfBirth, DateTime.Today) + ((adr.DateOfBirth.Month < DateTime.Today.Month || (adr.DateOfBirth.Day <= DateTime.Today.Day && adr.DateOfBirth.Month == DateTime.Today.Month)) ? 1 : 0)))).Take(15).ToList();

Or in a more readable format:

var query = from adr in context.Addresses
            where adr.DateOfBirth != null
            let diffYears = EntityFunctions.DiffYears(adr.DateOfBirth, DateTime.Today)
            let birthdayOccurred = adr.DateOfBirth.Month < DateTime.Today.Month || (adr.DateOfBirth.Day <= DateTime.Today.Day && adr.DateOfBirth.Month == DateTime.Today.Month)
            let nextBirthdate = EntityFunctions.AddYears(adr.DateOfBirth, diffYears + (birthdayOccurred ? 1 : 0))
            let daysToBirthdate = EntityFunctions.DiffDays(DateTime.Today, nextBirthdate)
            orderby daysToBirthdate
            select adr;

var result = query.Take(15).ToList();

Answer 2

I'm not sure you can do this as a one-liner. Certainly not with any degree of clarity.

The needed steps are:

1. Create an ordered list containing only the birthdates where the month/day comes after today.
2. Create an ordered list containing only the birthdates where the month/day is before today.
3. Append the second list to the first one, you now have a single list, sorted in birthday order.
4. Take the first 15.

I think the C# code would look something like this (You might need to add a List or two.)

var list1 = Addresses.Where(adr => adr.DateOfBirth != null && (adr.DateOfBirth.Value.Month > DateTime.Today.Month || (adr.DateOfBirth.Value.Month == DateTime.Today.Month && adr.DateOfBirth.Value.Day >= DateTime.Today.Day))).ToList();
var list2 = Addresses.Where(adr => adr.DateOfBirth != null && (adr.DateOfBirth.Value.Month < DateTime.Today.Month || (adr.DateOfBirth.Value.Month == DateTime.Today.Month && adr.DateOfBirth.Value.Day < DateTime.Today.Day))).ToList();
var fullList = list1.Add(list2);