Get the next date based on a collection of valid weekdays

Question

public static DateTime NextByDayOfWeek(this DateTime target, IEnumerable<DayOfWeek> validDaysOfWeek)
{
    validDaysOfWeek = validDaysOfWeek
        .OrderBy(item => item);

    var nextDayOfWeek = validDaysOfWeek
        .Where(item => item > target.DayOfWeek)
        .FirstOrDefault();

    return nextDayOfWeek != 0
        ? target.AddDays(nextDayOfWeek - target.DayOfWeek)
        : target.AddDays((validDaysOfWeek.First() + 7) - target.DayOfWeek);
}

Usage:

DateTime.Today.NextByDayOfWeek(new[] { DayOfWeek.Monday, DayOfWeek.Friday })

I don't think there are any possible edge cases here, at least some initial tests don't reveal any problem.

I think it's short and readable, but readability is often hard to judge when you're the author.

Any remarks?

Answer 1

I think a more readable way to write this would be to get the dates of the 7 days following the given date and return first one that matches the given days of week:

public static DateTime NextByDayOfWeek(
    this DateTime target, params DayOfWeek[] validDaysOfWeek)
{
    return Enumerable.Range(1, 7)
        .Select(n => target.AddDays(n))
        .First(date => validDaysOfWeek.Contains(date.DayOfWeek));
}

Notice that I also changes the validDaysOfWeek parameter to params DayOfWeek[], because that allows you to call the method like this:

DateTime.Today.NextByDayOfWeek(DayOfWeek.Monday, DayOfWeek.Friday)

If you also want the method to accept any IEnumerable<DayOfWeek>, you could add an overload for that.

Answer 2

You can just step forward one day at a time, until you find a week day that is valid. If you change the second parameter to a params parameter, as svick suggested, the code gets very straight forward:

public static DateTime NextByDayOfWeek(this DateTime target, params DayOfWeek[] validDaysOfWeek) {
  do {
    target = target.AddDays(1);
  } while (!validDaysOfWeek.Contains(target.DayOfWeek));
  return target;
}

You can also make a bitmask out of the valid days, so that you don't have to use the Contains method, which would loop through the collection of valid days for each iteration:

public static DateTime NextByDayOfWeek(this DateTime target, IEnumerable<DayOfWeek> validDaysOfWeek) {
  int valid = 0;
  foreach (DayOfWeek d in validDaysOfWeek) {
    valid |= 1 << (int)d;
  }
  do {
    target = target.AddDays(1);
  } while ((valid & 1 << (int)target.DayOfWeek) == 0);
  return target;
}

I tested both these functions with a range of different values, and they seem to give the correct result in all cases.