C# LINQ query to select only first date in a month from List<DateTime>

Question

I have a List which contains dates from June 1, 2009 to June 1, 2014. How would I query it in C# LINQ to select only the first date of each month?

Answer 1

The simplest way I think is to filter by the Day property of your dates:

var firstDays = dates.Where(d=> d.Day == 1);

Answer 2

Your question is slightly ambiguous. If you want the first item appearing in the list in each month, you can use:

var result = list.GroupBy(x => new { x.Year, x.Month })
                 .Select(x => x.Min());

Answer 3

Like this:

dates.Where(d => d.Day == 1);

Or, using query comprehension,

from d in dates where d.Day == 1 select d;

Answer 4

Try the following

public static bool IsFirstDayOfMonth(this DateTime t) {
  var other = new DateTime(t.Year,t.Month,1);
  return other == t.Date;
}

var allDates = GetTheDates();
var filter = allDates.Where(x => x.IsFirstDayOfMonth());