Calcule el último día de la semana de un mes

Question

Dado un DateTime y un DayOfWeek debería devolver la fecha del último DayOfWeek de ese mes.

E.g. 1 de marzo de 2009 y el domingo volvería 29 de marzo de 2009

Answer 1

No se puede encontrar una aplicación práctica de una sola línea, pero éste funciona:

static DateTime LastDayOfWeekInMonth(DateTime day, DayOfWeek dow) 
{ 
    DateTime lastDay = new DateTime(day.Year, day.Month, 1).AddMonths(1).AddDays(-1); 
    DayOfWeek lastDow = lastDay.DayOfWeek; 

    int diff = dow - lastDow; 

    if (diff > 0) diff -= 7; 

    System.Diagnostics.Debug.Assert(diff <= 0); 

    return lastDay.AddDays(diff); 
} 

Answer 2

Obtenga el primer día del mes siguiente, luego encuentre el primer día laborable de ese mes. Remontan a siete días y se encuentra en ese día de la semana en la última semana del mes Corrent:

DateTime date = new DateTime(2009, 3, 12); 
DayOfWeek day = DayOfWeek.Sunday; 

DateTime nextMonth = new DateTime(date.Year, date.Month, 1).AddMonths(1); 
while (nextMonth.DayOfWeek != day) { 
    nextMonth = nextMonth.AddDays(1); 
} 
DateTime lastInMonth = nextMonth.AddDays(-7); 

(podría reemplazar el bucle con un poco de aritmética que calcula el número de días a añadir sobre la base de los valores numéricos de la DayOfWeek valora, pero esto es más directo.)

Edit:
Por supuesto, también puede obtener el último día en el mes actual, luego retroceda hasta encontrar el día de la semana correcto.

Answer 3

http://datetimeextensions.codeplex.com/ tiene un (díadelasemana) Última método de extensión que va a hacer esto para usted

prueba incluyó :-)

Answer 4

Prueba esto: empezar por el último día y contar hacia atrás hasta llegar el día en que usted está buscando

static DateTime LastOccurenceOfDay(DateTime dt, DayOfWeek dow) 
    { 
     DateTime looperDate = new DateTime(dt.Year, dt.Month, 1) 
            .AddMonths(1).AddDays(-1); 
     while (looperDate.DayOfWeek!=dow)        
      looperDate =looperDate.AddDays(-1);    
     return looperDate; 

Editar: Se ha recuperado la versión anterior a la que se refiere Dangph en los comentarios.

static DateTime LastOccurenceOfDay(DateTime dt, DayOfWeek dow) 
    { 
     //set start of loop 
     DateTime looperDate = new DateTime(dt.Year, dt.Month, 1); 
     //initialze return value 
     DateTime returnDate = dt; 
     //loop until the month is over 
     while (looperDate.Month == dt.Month) 
     { 
      //if the current DayOfWeek is the date you're looking for 
      if (looperDate.DayOfWeek == dow) 
       //remember it. 
       returnDate = looperDate; 
      //increase day 
      looperDate=looperDate.AddDays(1); 
     } 
     return returnDate; 
    } 

Answer 5

static DateTime LastDateOfWeekForMonth(DayOfWeek weekday, int month, int year) 
{ 
    DateTime d = new DateTime(year, month, 1).AddMonths(1); 
    while (!(d.DayOfWeek == weekday && d.Month == month)) 
    { 
     d = d.AddDays(-1); 
    } 
    return d; 
} 

Answer 6

public DateTime GetLastDayOfMonth(int year, int month, DayOfWeek dayOfWeek) 
    { 
     var daysInMonth = DateTime.DaysInMonth(year, month); 
     var lastDay = new DateTime(year, month, daysInMonth); 

     while (lastDay.DayOfWeek != dayOfWeek) 
     { 
      lastDay = lastDay.AddDays(-1); 
     } 

     return lastDay; 
    } 

Answer 7

static DateTime GetDayOfWeekInMonthFromWeekIndex(this DateTime date, DayOfWeek dow, int weekIndex) 
{ 
    if (weekIndex > 4) 
     return LastDayOfWeekInMonth(date, dow); 

    DateTime newDate = new DateTime(date.Year, date.Month, 1); 
    DayOfWeek newDow = newDate.DayOfWeek; 
    int diff = dow - newDow; 
    diff += ((weekIndex - 1) * 7); 
    return newDate.AddDays(diff); 
} 

Answer 8

Aquí es tanto LastDayOfMonth() y LastDayOfMonth (DayOfWeek) más unidad pruebas, que se inspira en la aplicación "Última Viernes" en How to find the 3rd Friday in a month with C#? por @ Pablo freidora:

/** 
    /// <summary>An extension method that returns the last day of the month.</summary> 
    /// <param name="d">A date within the month to calculate.</param> 
    /// <returns>The last day of the current month.</returns> 
    **/ 
    public static DateTime LastDayOfMonth(this DateTime d) 
    { 
     DateTime nextMonth = new DateTime(d.Year, d.Month, 1).AddMonths(1); 
     return nextMonth.AddDays(-1); 
    } 

    /** 
    /// <summary>An extension method that returns the last <see cref="DayOfWeek"> of the month.</summary> 
    /// <param name="d">A date within the month to calculate.</param> 
    /// <returns>The last day of the current month.</returns> 
    **/ 
    public static DateTime LastDayOfMonth(this DateTime d, DayOfWeek dayOfWeek) 
    { 
     DateTime lastDayOfMonth = d.LastDayOfMonth(); 
     int vector = (((int)lastDayOfMonth.DayOfWeek - (int)dayOfWeek + DaysInWeek) % DaysInWeek); 
     return lastDayOfMonth.AddDays(-vector); 
    } 

#region LastDayOfMonth Tests 

    [TestCase("1/1/2011", "1/31/2011")] 
    [TestCase("2/1/2009", "2/28/2009")] //Leap Year 
    [TestCase("2/1/2008", "2/29/2008")] //Leap Year 
    [TestCase("3/10/2011", "3/31/2011")] 
    [TestCase("4/20/2011", "4/30/2011")] 
    [TestCase("5/15/2011", "5/31/2011")] 
    [TestCase("6/30/2011", "6/30/2011")] 
    [TestCase("12/1/2011", "12/31/2011")] 
    public void LastDayOfMonthReturnsCorrectDay(string startingDate, string expectedDate) 
    { 
     //Arrange 
     DateTime testDate = DateTime.Parse(startingDate); 
     DateTime expected = DateTime.Parse(expectedDate); 

     //Act 
     DateTime actual = testDate.LastDayOfMonth(); 

     //Assert 
     Assert.That(actual, Is.EqualTo(expected)); 
    } 

    [TestCase("1/1/2011", DayOfWeek.Monday, "1/31/2011")] 
    [TestCase("2/1/2009", DayOfWeek.Saturday, "2/28/2009")] 
    [TestCase("2/1/2009", DayOfWeek.Sunday, "2/22/2009")] 
    [TestCase("2/1/2008", DayOfWeek.Friday, "2/29/2008")] //Leap Year 
    [TestCase("2/1/2008", DayOfWeek.Thursday, "2/28/2008")] //Leap Year 
    [TestCase("3/10/2011", DayOfWeek.Wednesday, "3/30/2011")] 
    [TestCase("4/20/2011", DayOfWeek.Friday, "4/29/2011")] 
    [TestCase("4/20/2011", DayOfWeek.Saturday, "4/30/2011")] 
    [TestCase("5/15/2011", DayOfWeek.Tuesday, "5/31/2011")] 
    [TestCase("5/15/2011", DayOfWeek.Saturday, "5/28/2011")] 
    [TestCase("9/30/2011", DayOfWeek.Sunday, "9/25/2011")] 
    [TestCase("12/1/2011", DayOfWeek.Monday, "12/26/2011")] 
    public void LastDayOfMonthReturnsCorrectDayOfWeek(string startingDate, DayOfWeek dayOfWeek, string expectedDate) 
    { 
     //Arrange 
     DateTime testDate = DateTime.Parse(startingDate); 
     DateTime expected = DateTime.Parse(expectedDate); 

     //Act 
     DateTime actual = testDate.LastDayOfMonth(dayOfWeek); 

     //Assert 
     Assert.That(actual, Is.EqualTo(expected)); 
    }  

    #endregion