How do I loop through a date range?

I'm not even sure how to do this without using some horrible for loop/counter type solution. Here's the problem:

I'm given two dates, a start date and an end date and on a specified interval I need to take some action. For example: for every date between 3/10/2009 on every third day until 3/26/2009 I need to create an entry in a List. So my inputs would be:

DateTime StartDate = "3/10/2009";
DateTime EndDate = "3/26/2009";
int DayInterval = 3;

and my output would be a list that has the following dates:

3/13/2009 3/16/2009 3/19/2009 3/22/2009 3/25/2009

So how the heck would I do something like this? I thought about using a for loop that would iterate between every day in the range with a separate counter like so:

int count = 0;

for(int i = 0; i < n; i++)
{
     count++;
     if(count >= DayInterval)
     {
          //take action
          count = 0;
     }

}

But it seems like there could be a better way?

Well, you'll need to loop over them one way or the other. I prefer defining a method like this:

public IEnumerable<DateTime> EachDay(DateTime from, DateTime thru)
{
    for(var day = from.Date; day.Date <= thru.Date; day = day.AddDays(1))
        yield return day;
}

Then you can use it like this:

foreach (DateTime day in EachDay(StartDate, EndDate))
    // print it or whatever

In this manner you could hit every other day, every third day, only weekdays, etc. For example, to return every third day starting with the "start" date, you could just call AddDays(3) in the loop instead of AddDays(1).

I have a Range class in MiscUtil which you could find useful. Combined with the various extension methods, you could do:

foreach (DateTime date in StartDate.To(EndDate).ExcludeEnd()
                                   .Step(DayInterval.Days())
{
    // Do something with the date
}

(You may or may not want to exclude the end - I just thought I'd provide it as an example.)

This is basically a ready-rolled (and more general-purpose) form of mquander's solution.

For your example you can try

DateTime StartDate = new DateTime(2009, 3, 10);
DateTime EndDate = new DateTime(2009, 3, 26);
int DayInterval = 3;

List<DateTime> dateList = new List<DateTime>();
while (StartDate.AddDays(DayInterval) <= EndDate)
{
   StartDate = StartDate.AddDays(DayInterval);
   dateList.Add(StartDate);
}

Code from @mquander and @Yogurt The Wise used in extensions:

public static IEnumerable<DateTime> EachDay(DateTime from, DateTime thru)
{
    for (var day = from.Date; day.Date <= thru.Date; day = day.AddDays(1))
        yield return day;
}

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

public static IEnumerable<DateTime> EachDayTo(this DateTime dateFrom, DateTime dateTo)
{
    return EachDay(dateFrom, dateTo);
}

public static IEnumerable<DateTime> EachMonthTo(this DateTime dateFrom, DateTime dateTo)
{
    return EachMonth(dateFrom, dateTo);
}

1 Year later, may it help someone,

This version includes a predicate, to be more flexible.

Usage

var today = DateTime.UtcNow;
var birthday = new DateTime(2018, 01, 01);

Daily to my birthday

var toBirthday = today.RangeTo(birthday);

Monthly to my birthday, Step 2 months

var toBirthday = today.RangeTo(birthday, x => x.AddMonths(2));

Yearly to my birthday

var toBirthday = today.RangeTo(birthday, x => x.AddYears(1));

Use `RangeFrom` instead

// same result
var fromToday = birthday.RangeFrom(today);
var toBirthday = today.RangeTo(birthday);

Implementation

public static class DateTimeExtensions 
{

    public static IEnumerable<DateTime> RangeTo(this DateTime from, DateTime to, Func<DateTime, DateTime> step = null)
    {
        if (step == null)
        {
            step = x => x.AddDays(1);
        }

        while (from < to)
        {
            yield return from;
            from = step(from);
        }
    }

    public static IEnumerable<DateTime> RangeFrom(this DateTime to, DateTime from, Func<DateTime, DateTime> step = null)
    {
        return from.RangeTo(to, step);
    }
}