Claudio Lassala's Blog

Here’s something fun with can do with C#. Imagine we need to work with dates within a given year. For instance, I want to list out all days within the year of 2017. A little Calendar class like the one below would be handy:

var calendar = new Calendar(2017);
var days = calendar;
foreach (var day in days)
{
    Console.WriteLine(day);
}

The output looks somewhat like this:

What if I wanted to list all days in january, like so:

var days = calendar. Where(d => d.Month == 1);

The output:

Last but not least, maybe I want to list all Saturdays in january:

var days = calendar.Where(d => d.Month == 1 && d.DayOfWeek == DayOfWeek.Saturday))

The output:

One could think that under the hood the Calendar class uses some sort of Collection, Array or List of DateTime objects. In reality, it uses an iterator, implemented using the yield return keywords:

public class Calendar : IEnumerable<DateTime>
{
    readonly int _year;

    public Calendar(int year)
    {
        _year = year;
    }
    public IEnumerator<DateTime> GetEnumerator()
    {
        var firstDayOfTheYear = new DateTime(_year, 1, 1);
        var lastDayOfTheYear = new DateTime(_year, 12, 31);
        var daysInTheYear = (lastDayOfTheYear - firstDayOfTheYear).TotalDays;
        for (int i = 0; i <= daysInTheYear; i++)
        {
            yield return firstDayOfTheYear.AddDays(i);             
        }
    }
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Ain’t that fun and handy? 🙂

I really like this combination of IEnumerable<T>, iterators, and LINQ.