/// <summary>
/// Finds the total number of instances of a specific DayOfWeek in a month.
/// </summary>
/// <param name="dayOfWeek">The day of week.</param>
/// <param name="year">The year.</param>
/// <param name="month">The month.</param>
/// <returns></returns>
public static int TotalInstancesInMonth(
this DayOfWeek dayOfWeek,
int year,
int month)
{
return(DateTimeExtensions.DaysOfMonth(year, month).Count(date => dayOfWeek.Equals(date.DayOfWeek)));
}
/// <summary>
/// Finds the NTH week day of a month.
/// </summary>
/// <param name="dayOfWeek">The day of week.</param>
/// <param name="year">The year.</param>
/// <param name="month">The month.</param>
/// <param name="n">The nth instance.</param>
/// <remarks>Compensates for 4th and 5th DayOfWeek of Month</remarks>
public static DateTime FindNthWeekDayOfMonth(
this DayOfWeek dayOfWeek,
int year,
int month,
int n)
{
if (n is < 1 or > 5)
{
throw new ArgumentOutOfRangeException(nameof(n));
}
var y = 0;
IEnumerable <DateTime> daysOfMonth = DateTimeExtensions.DaysOfMonth(year, month);
// compensate for "last DayOfWeek in month"
var totalInstances = dayOfWeek.TotalInstancesInMonth(year, month);
if (n == 5 &&
n > totalInstances)
{
n = 4;
}
var foundDate = daysOfMonth
.Where(date => dayOfWeek.Equals(date.DayOfWeek))
.OrderBy(date => date)
.Select(x => new { n = ++y, date = x })
.Where(x => x.n.Equals(n))
.Select(x => x.date)
.First(); //black magic wizardry
return(foundDate);
}
public void DaysOfMonth_Test()
{
var days = DateTimeExtensions.DaysOfMonth(2018, 1);
days.ShouldNotBeNull();
days.Count().ShouldBe(31);
}
public static DateTime NthDayOfMonthFrom(
this List <DayOfWeek> daysOfWeek,
int year,
int month,
int nth
)
{
return
(DateTimeExtensions.DaysOfMonth(year, month)
.Where(
date => daysOfWeek.Contains(date.DayOfWeek)
)
.Skip(nth - 1)
.First());
}