/// <summary>
/// Converts a specified <c>ISO-8601</c> year, week of the year and day of the week to its <see cref="DateTime"/>
/// equivalent value.
/// </summary>
/// <param name="year">The year.</param>
/// <param name="week">The week of the year.</param>
/// <param name="dayOfWeek">The day of the week.</param>
/// <returns>
/// A <see cref="DateTime"/> value that is equivalent to the specified <c>ISO-8601</c> year,
/// week of the year and day of the week.
/// </returns>
/// <exception cref="ArgumentOutOfRangeException">
/// <paramref name="year"/> is less than 1 or greater than 9999. Or,
/// <paramref name="week"/> is less than 1 or greater than 53.
/// </exception>
public static DateTime DateForIso8601(int year, int week, DayOfWeek dayOfWeek)
{
Contract.Requires <ArgumentOutOfRangeException>(year >= 1 && year <= 9999, nameof(year));
Contract.Requires <ArgumentOutOfRangeException>(week >= 1 && week <= 53, nameof(week));
var firstDayOfYear = new DateTime(year, 1, 1);
var firstDayOfYearFromStartOfWeek = firstDayOfYear.DaysFromStartOfTheWeek(DayOfWeek.Monday);
var daysInWholeWeeks = (week - 1) * DaysPerWeek;
var daysFromStartOfYear = dayOfWeek.DaysFromStartOfTheWeek() - firstDayOfYearFromStartOfWeek + daysInWholeWeeks;
if (firstDayOfYearFromStartOfWeek > 3)
{
daysFromStartOfYear += DaysPerWeek;
}
return(firstDayOfYear.AddDays(daysFromStartOfYear));
}
/// <summary>
/// Returns the date for a specific occurrence of a given day of the week in a specified month of a the year.
/// </summary>
/// <param name="weekOfMonth">The week instance of the month.</param>
/// <param name="dayOfWeek">The day of the week.</param>
/// <param name="month">The month of the year.</param>
/// <param name="year">The year.</param>
/// <returns>A <see cref="DateTime"/> value.</returns>
/// <exception cref="ArgumentOutOfRangeException">
/// <paramref name="year"/> is less than 1 or greater than 9999. Or,
/// <paramref name="month"/> is less than 1 or greater than 12.
/// </exception>
public static DateTime DateFor(WeekOfMonth weekOfMonth, DayOfWeek dayOfWeek, int month, int year)
{
Contract.Requires <ArgumentOutOfRangeException>(year >= 1 && year <= 9999, nameof(year));
Contract.Requires <ArgumentOutOfRangeException>(month >= 1 && month <= 12, nameof(month));
var firstOfMonth = new DateTime(year, month, 1);
var daysInWholeWeeks = ((int)weekOfMonth - 1) * DaysPerWeek;
var daysFromStartOfMonth = dayOfWeek.DaysFromStartOfTheWeek() - firstOfMonth.DayOfWeek.DaysFromStartOfTheWeek();
if (daysFromStartOfMonth < 0)
{
daysFromStartOfMonth += DaysPerWeek;
}
daysFromStartOfMonth += daysInWholeWeeks;
if (weekOfMonth == WeekOfMonth.Last && daysFromStartOfMonth >= DateTime.DaysInMonth(year, month))
{
daysFromStartOfMonth -= DaysPerWeek;
}
return(firstOfMonth.AddDays(daysFromStartOfMonth));
}
/// <summary>
/// Converts a <see cref="DayOfWeek"/> value to a <see cref="DaysOfTheWeek"/> value.
/// </summary>
/// <param name="source">The day of the week.</param>
/// <returns>A <see cref="DaysOfTheWeek"/> value.</returns>
public static DaysOfTheWeek ToDaysOfWeek(this DayOfWeek source)
{
return((DaysOfTheWeek)(1 << source.DaysFromStartOfTheWeek()));
}
public int DaysFromStartOfTheWeek(DayOfWeek firstDayOfWeek, DayOfWeek dayOfWeek)
{
return(dayOfWeek.DaysFromStartOfTheWeek(firstDayOfWeek));
}