Currently I fetch all customers that have registered exactly 2 years before a given date.

var observationPeriod = DateTime.Today;
var observedDate = observationPeriod.AddYears(-2);

var customers = unitOfWork.CustomerRepository
                                .GetAll()
                                .Where(o => o.RegistrationDate.Date == observedDate)
                                .ToList();

But my requirement is to get all customers that registered 2x years before, e.g. 2, 4, 6, etc. years. Ideally, this should also be entity framework core compatible. What would you recommend?

Currently I fetch all customers that have registered exactly 2 years before a given date.

var observationPeriod = DateTime.Today;
var observedDate = observationPeriod.AddYears(-2);

var customers = unitOfWork.CustomerRepository
                                .GetAll()
                                .Where(o => o.RegistrationDate.Date == observedDate)
                                .ToList();

But my requirement is to get all customers that registered 2x years before, e.g. 2, 4, 6, etc. years. Ideally, this should also be entity framework core compatible. What would you recommend?

• c#
• linq-to-sql

• 1 How far back do you need to go? If you can set a limit, say 50years, you could probably just create a list of all the dates and do a .Contains(...) – JonasH Commented Mar 13 at 9:41
• yes I thought about that, if there is no generic solution I will go this way – AGuyCalledGerald Commented Mar 13 at 9:42
• 1 Isn't this an infinite list of dates if you dont limit it. I mean not absolutely infinite but for all intents it is. – Jamiec Commented Mar 13 at 9:47
• What does .GetAll return? Is it an IEnumerable or IQueryable? If IEnumerable realize that your database will return the entire table and any additional filters you apply are in memory which may not be what you expect. – Jim Wooley Commented Mar 13 at 20:15
• 1 Do you really use LINQ-to-SQL? It's probably entity framework core. If so, please retag and reword the question. – Gert Arnold Commented Mar 14 at 18:32

1 Answer 1

You can check if the difference between the registration date and the observation period is a multiple of 2 years

var observationPeriod = DateTime.Today;

var customers = unitOfWork.CustomerRepository
   .GetAll()
   .Where(o => (observationPeriod.Year - o.RegistrationDate.Year) % 2 == 0 &&
              o.RegistrationDate.Date == observationPeriod.AddYears(-(observationPeriod.Year - o.RegistrationDate.Year)).Date &&
              o.RegistrationDate <= observationPeriod)
   .ToList();