Select Only Fourth Sunday of each month

I am stuck for sometime now, now need your help.

I want开发者_运维百科 to display in a dropdown only fourth Sunday of each month, say from 1-Sep-2010 to 31-Aug-2011

I only want fourth Sunday in dropdown list, how to do it using asp.net C#

Regards

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Here is an approach that uses a little LINQ and the knowledge that the fourth Sunday will occur between the 22nd and 28th of a month, inclusive.

DateTime startDate = new DateTime(2010, 9, 1);
DateTime endDate = startDate.AddYears(1).AddDays(-1);

List<DateTime> fourthSundays = new List<DateTime>();

DateTime currentDate = startDate;
while (currentDate < endDate)
{
    // we know the fourth sunday will be the 22-28
    DateTime fourthSunday = Enumerable.Range(22, 7).Select(day => new DateTime(currentDate.Year, currentDate.Month, day)).Single(date => date.DayOfWeek == DayOfWeek.Sunday);
    fourthSundays.Add(fourthSunday);
    currentDate = currentDate.AddMonths(1);
}

You can then bind that List<DateTime> to the dropdown or skip the list itself in favor of adding the items as you generate them to the dropdown, like below.

yourDropdown.Items.Add(new ListItem(fourthSunday.ToString()));

For giggles, you can do the whole thing in a LINQ statement and skip (most of) the variables.

DateTime startDate = new DateTime(2010, 9, 1); 
IEnumerable<DateTime> fourthSundays =
    Enumerable.Range(0, 12)
    .Select(item => startDate.AddMonths(item))
    .Select(currentMonth =>
        Enumerable.Range(22, 7)
        .Select(day => new DateTime(currentMonth.Year, currentMonth.Month, day))
        .Single(date => date.DayOfWeek == DayOfWeek.Sunday)
    );

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Got bored so here you go. Two helper methods one retrieves the Week if it exist, and the other iterates through the months

class Program
{
    static void Main(string[] args)
    {
        DateTime startDate = new DateTime(2010, 09, 1);
        foreach(DateTime dt in EachMonth( new DateTime(2010, 09, 1), new DateTime(2011, 09, 1))){
            DateTime? result = GetDayByWeekOffset(DayOfWeek.Sunday, dt, 4);
           Console.WriteLine("Sunday:" + (result.HasValue?result.Value.ToString("MM-dd-yyyy"):"null"));
        }
        Console.ReadKey();
    }

    public static DateTime? GetDayByWeekOffset(DayOfWeek day, DateTime month, int weekOffSet)
    {
        //First day of month
        DateTime firstDayOfMonth = month.AddDays((-1 * month.Day) + 1);

        // 
        int daysOffSet;
        daysOffSet= ((int)day + 7 - (int)firstDayOfMonth.DayOfWeek) % 7;
        DateTime firstDay = month.AddDays(daysOffSet);

        // Add the number of weeks specified
        DateTime resultDate = firstDay.AddDays((weekOffSet - 1) * 7);

        if (resultDate.Month != firstDayOfMonth.Month){
            return null;
        }else{
            return resultDate;
        }
    }

    public static IEnumerable<DateTime> EachMonth(DateTime from, DateTime thru)
    {
        for (var month = from.Date; month.Date <= thru.Date; month = month.AddMonths(1))
            yield return month;
    }
}

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Anthony's answer above is nice, I like it a lot. As an alternate, here is a method which is parameterized for the day of the week and the week number (i.e. if you need other combinations, like 4th Sunday, 3rd Friday, etc.) with some comments.

Call it like this for your case:

List<DateTime> sundays = DateInstances(new DateTime(2010, 9, 1), new DateTime(2011, 8, 31), DayOfWeek.Sunday, 4);

And the method itself:

public List<DateTime> DateInstances(DateTime start, DateTime end, DayOfWeek day, int weeks)
{
    if (start > end)
        throw new ArgumentOutOfRangeException("end", "The start date must occur before the end date");

    List<DateTime> results = new List<DateTime>();

    DateTime temp = start;
    while (temp < end)
    {
        DateTime firstWeekday = new DateTime(temp.Year, temp.Month, 1);

        //increment to the given day (i.e. if we want the 4th sunday, we must find the first sunday of the month)
        while (firstWeekday.DayOfWeek != day)
            firstWeekday = firstWeekday.AddDays(1);

        //add the number of weeks (note: we already have the first instance, so subtract 1)
        firstWeekday = firstWeekday.AddDays(7 * (weeks - 1));

        //make sure we haven't gone over to the next month
        if (firstWeekday.Month == temp.Month)
            results.Add(firstWeekday);

        //let's not loop forever ;)
        temp = temp.AddMonths(1);
    }

    return results;
}