我如何获取星期初的DateTime？

我如何只知道C＃中的当前时间来查找一周的开始（星期日和星期一）？

就像是：

DateTime.Now.StartWeek(Monday);

使用扩展方法。他们是一切的答案，你知道的！;）

public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(-1 * diff).Date;
    }
}

可以如下使用：

DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Monday);
DateTime dt = DateTime.Now.StartOfWeek(DayOfWeek.Sunday);

我想出的最快方法是：

var sunday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek);

如果您希望将一周中的其他日期作为开始日期，则只需将DayOfWeek值添加到结束位置即可

var monday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek + (int)DayOfWeek.Monday);

var tuesday = DateTime.Today.AddDays(-(int)DateTime.Today.DayOfWeek + (int)DayOfWeek.Tuesday); 

更详细和文化意识：

System.Globalization.CultureInfo ci = 
    System.Threading.Thread.CurrentThread.CurrentCulture;
DayOfWeek fdow = ci.DateTimeFormat.FirstDayOfWeek;
DayOfWeek today = DateTime.Now.DayOfWeek;
DateTime sow = DateTime.Now.AddDays(-(today - fdow)).Date;

使用Fluent DateTime：

var monday = DateTime.Now.Previous(DayOfWeek.Monday);
var sunday = DateTime.Now.Previous(DayOfWeek.Sunday);

丑陋，但至少可以给您正确的日期

通过系统设置星期开始时间：

    public static DateTime FirstDateInWeek(this DateTime dt)
    {
        while (dt.DayOfWeek != System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek)
            dt = dt.AddDays(-1);
        return dt;
    }

没有：

    public static DateTime FirstDateInWeek(this DateTime dt, DayOfWeek weekStartDay)
    {
        while (dt.DayOfWeek != weekStartDay)
            dt = dt.AddDays(-1);
        return dt;
    }

让我们结合文化安全性答案和扩展方法答案：

public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
        DayOfWeek fdow = ci.DateTimeFormat.FirstDayOfWeek;
        return DateTime.Today.AddDays(-(DateTime.Today.DayOfWeek- fdow));
    }
}

这将为您提供前一个星期日（我认为）：

DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek, 0, 0, 0);

这可能有点hack，但是您可以将.DayOfWeek属性强制转换为int（这是一个枚举，并且由于其底层数据类型没有更改，因此默认为int），并使用该值来确定本周的上一个开始。

似乎在DayOfWeek枚举中指定的星期从星期日开始，因此，如果我们从该值中减去1，则等于星期一在当前日期之前的天数。我们还需要将星期日（0）映射为等于7，因此给定1-7 = -6，则星期日将映射到上一个星期一：

DateTime now = DateTime.Now;
int dayOfWeek = (int)now.DayOfWeek;
dayOfWeek = dayOfWeek == 0 ? 7 : dayOfWeek;
DateTime startOfWeek = now.AddDays(1 - (int)now.DayOfWeek);

上一个星期日的代码更简单，因为我们不必进行此调整：

DateTime now = DateTime.Now;
int dayOfWeek = (int)now.DayOfWeek;
DateTime startOfWeek = now.AddDays(-(int)now.DayOfWeek);

对于星期一

DateTime startAtMonday = DateTime.Now.AddDays(DayOfWeek.Monday - DateTime.Now.DayOfWeek);

周日

DateTime startAtSunday = DateTime.Now.AddDays(DayOfWeek.Sunday- DateTime.Now.DayOfWeek);

using System;
using System.Globalization;

namespace MySpace
{
    public static class DateTimeExtention
    {
        // ToDo: Need to provide culturaly neutral versions.

        public static DateTime GetStartOfWeek(this DateTime dt)
        {
            DateTime ndt = dt.Subtract(TimeSpan.FromDays((int)dt.DayOfWeek));
            return new DateTime(ndt.Year, ndt.Month, ndt.Day, 0, 0, 0, 0);
        }

        public static DateTime GetEndOfWeek(this DateTime dt)
        {
            DateTime ndt = dt.GetStartOfWeek().AddDays(6);
            return new DateTime(ndt.Year, ndt.Month, ndt.Day, 23, 59, 59, 999);
        }

        public static DateTime GetStartOfWeek(this DateTime dt, int year, int week)
        {
            DateTime dayInWeek = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
            return dayInWeek.GetStartOfWeek();
        }

        public static DateTime GetEndOfWeek(this DateTime dt, int year, int week)
        {
            DateTime dayInWeek = new DateTime(year, 1, 1).AddDays((week - 1) * 7);
            return dayInWeek.GetEndOfWeek();
        }
    }
}

通过全球化将它们放在一起，并允许指定星期几作为电话会议的一部分

public static DateTime StartOfWeek ( this DateTime dt, DayOfWeek? firstDayOfWeek )
{
    DayOfWeek fdow;

    if ( firstDayOfWeek.HasValue  )
    {
        fdow = firstDayOfWeek.Value;
    }
    else
    {
        System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
        fdow = ci.DateTimeFormat.FirstDayOfWeek;
    }

    int diff = dt.DayOfWeek - fdow;

    if ( diff < 0 )
    {
        diff += 7;
    }

    return dt.AddDays( -1 * diff ).Date;

}

var now = System.DateTime.Now;

var result = now.AddDays(-((now.DayOfWeek - System.Threading.Thread.CurrentThread.CurrentCulture.DateTimeFormat.FirstDayOfWeek + 7) % 7)).Date;

这将使您在一周的第一个星期日的午夜：

DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek, t.Hour, t.Minute, t.Second);

这使您在午夜的第一个星期一：

DateTime t = DateTime.Now;
t -= new TimeSpan ((int) t.DayOfWeek - 1, t.Hour, t.Minute, t.Second);

在C＃中尝试使用此代码，您可以获取给定星期的开始日期和结束日期，这里星期日是第一天，星期六是最后一天，但是您可以根据自己的文化设置两天的日期

DateTime firstDate = GetFirstDateOfWeek(DateTime.Parse("05/09/2012").Date,DayOfWeek.Sunday);
DateTime lastDate = GetLastDateOfWeek(DateTime.Parse("05/09/2012").Date, DayOfWeek.Saturday);

public static DateTime GetFirstDateOfWeek(DateTime dayInWeek, DayOfWeek firstDay)
{
    DateTime firstDayInWeek = dayInWeek.Date;
    while (firstDayInWeek.DayOfWeek != firstDay)
        firstDayInWeek = firstDayInWeek.AddDays(-1);

    return firstDayInWeek;
}
public static DateTime GetLastDateOfWeek(DateTime dayInWeek, DayOfWeek firstDay)
{
    DateTime lastDayInWeek = dayInWeek.Date;
    while (lastDayInWeek.DayOfWeek != firstDay)
        lastDayInWeek = lastDayInWeek.AddDays(1);

    return lastDayInWeek;
}

尝试了几次，但从星期一开始的一个星期后仍未解决问题，导致在星期日的下周一给了我。因此，我对其进行了一些修改并使其与以下代码一起使用：

int delta = DayOfWeek.Monday - DateTime.Now.DayOfWeek;
DateTime monday = DateTime.Now.AddDays(delta == 1 ? -6 : delta);
return monday;

步骤1：建立静态类别

  public static class TIMEE
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = (7 + (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(-1 * diff).Date;
    }
    public static DateTime EndOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = (7 - (dt.DayOfWeek - startOfWeek)) % 7;
        return dt.AddDays(1 * diff).Date;
    }
}

步骤2：使用此类来获取一周的开始和结束日期

 DateTime dt =TIMEE.StartOfWeek(DateTime.Now ,DayOfWeek.Monday);
        DateTime dt1 = TIMEE.EndOfWeek(DateTime.Now, DayOfWeek.Sunday);

下面的方法应返回所需的DateTime。如果星期天是一周的第一天，则传递true，星期一传递false：

private DateTime getStartOfWeek(bool useSunday)
{
    DateTime now = DateTime.Now;
    int dayOfWeek = (int)now.DayOfWeek;

    if(!useSunday)
        dayOfWeek--;

    if(dayOfWeek < 0)
    {// day of week is Sunday and we want to use Monday as the start of the week
    // Sunday is now the seventh day of the week
        dayOfWeek = 6;
    }

    return now.AddDays(-1 * (double)dayOfWeek);
}

感谢您的示例。我需要始终在一周的第一天使用“ CurrentCulture”，对于一个数组，我需要知道确切的Daynumber ..所以这是我的第一个扩展：

public static class DateTimeExtensions
{
    //http://stackoverflow.com/questions/38039/how-can-i-get-the-datetime-for-the-start-of-the-week
    //http://stackoverflow.com/questions/1788508/calculate-date-with-monday-as-dayofweek1
    public static DateTime StartOfWeek(this DateTime dt)
    {
        //difference in days
        int diff = (int)dt.DayOfWeek - (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek; //sunday=always0, monday=always1, etc.

        //As a result we need to have day 0,1,2,3,4,5,6 
        if (diff < 0)
        {
            diff += 7;
        }
        return dt.AddDays(-1 * diff).Date;
    }

    public static int DayNoOfWeek(this DateTime dt)
    {
        //difference in days
        int diff = (int)dt.DayOfWeek - (int)CultureInfo.CurrentCulture.DateTimeFormat.FirstDayOfWeek; //sunday=always0, monday=always1, etc.

        //As a result we need to have day 0,1,2,3,4,5,6 
        if (diff < 0)
        {
            diff += 7;
        }
        return diff + 1; //Make it 1..7
    }
}

似乎没有人正确回答这个问题。如果有人需要，我将在此处粘贴我的解决方案。不管一周的第一天是星期一，星期日还是其他时间，下面的代码都可以工作。

public static class DateTimeExtension
{
  public static DateTime GetFirstDayOfThisWeek(this DateTime d)
  {
    CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
    var first = (int)ci.DateTimeFormat.FirstDayOfWeek;
    var current = (int)d.DayOfWeek;

    var result = first <= current ?
      d.AddDays(-1 * (current - first)) :
      d.AddDays(first - current - 7);

    return result;
  }
}

class Program
{
  static void Main()
  {
    System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("en-US");
    Console.WriteLine("Current culture set to en-US");
    RunTests();
    Console.WriteLine();
    System.Threading.Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo("da-DK");
    Console.WriteLine("Current culture set to da-DK");
    RunTests();
    Console.ReadLine();
  }

  static void RunTests()
  {
    Console.WriteLine("Today {1}: {0}", DateTime.Today.Date.GetFirstDayOfThisWeek(), DateTime.Today.Date.ToString("yyyy-MM-dd"));
    Console.WriteLine("Saturday 2013-03-02: {0}", new DateTime(2013, 3, 2).GetFirstDayOfThisWeek());
    Console.WriteLine("Sunday 2013-03-03: {0}", new DateTime(2013, 3, 3).GetFirstDayOfThisWeek());
    Console.WriteLine("Monday 2013-03-04: {0}", new DateTime(2013, 3, 4).GetFirstDayOfThisWeek());
  }
}

C＃中的Modulo不适用于-1mod7（应为6，C＃返回-1），因此...“单一”解决方案如下所示：）

private static DateTime GetFirstDayOfWeek(DateTime date)
    {
        return date.AddDays(-(((int)date.DayOfWeek - 1) - (int)Math.Floor((double)((int)date.DayOfWeek - 1) / 7) * 7));
    }

周末也一样（以@Compile的形式给出）：

    public static DateTime EndOfWeek(this DateTime dt)
    {
        int diff = 7 - (int)dt.DayOfWeek;

        diff = diff == 7 ? 0 : diff;

        DateTime eow = dt.AddDays(diff).Date;

        return new DateTime(eow.Year, eow.Month, eow.Day, 23, 59, 59, 999) { };
    }

您可以使用出色的Umbrella库：

using nVentive.Umbrella.Extensions.Calendar;
DateTime beginning = DateTime.Now.BeginningOfWeek();

但是，它们似乎确实将星期一存储为一周的第一天（请参阅属性nVentive.Umbrella.Extensions.Calendar.DefaultDateTimeCalendarExtensions.WeekBeginsOn），因此以前的本地化解决方案要好一些。不幸的。

编辑：仔细研究这个问题，看起来伞实际上也可以解决这个问题：

// Or DateTime.Now.PreviousDay(DayOfWeek.Monday)
DateTime monday = DateTime.Now.PreviousMonday(); 
DateTime sunday = DateTime.Now.PreviousSunday();

尽管值得注意的是，如果您在星期一要求上一个星期一，它将给您7天的时间。但是，如果您使用BeginningOfWeek，也是如此，这似乎是一个错误:(。

这将返回一周的开始和结束日期：

    private string[] GetWeekRange(DateTime dateToCheck)
    {
        string[] result = new string[2];
        TimeSpan duration = new TimeSpan(0, 0, 0, 0); //One day 
        DateTime dateRangeBegin = dateToCheck;
        DateTime dateRangeEnd = DateTime.Today.Add(duration);

        dateRangeBegin = dateToCheck.AddDays(-(int)dateToCheck.DayOfWeek);
        dateRangeEnd = dateToCheck.AddDays(6 - (int)dateToCheck.DayOfWeek);

        result[0] = dateRangeBegin.Date.ToString();
        result[1] = dateRangeEnd.Date.ToString();
        return result;

    }

我已经在我的博客ZamirsBlog上发布了用于计算周，月，季度和年的开始/结束的完整代码。

    namespace DateTimeExample
    {
        using System;

        public static class DateTimeExtension
        {
            public static DateTime GetMonday(this DateTime time)
            {
                if (time.DayOfWeek != DayOfWeek.Monday)
                    return GetMonday(time.AddDays(-1)); //Recursive call

                return time;
            }
        }

        internal class Program
        {
            private static void Main()
            {
                Console.WriteLine(DateTime.Now.GetMonday());
                Console.ReadLine();
            }
        }
    } 

这是一些答案的组合。它使用一种扩展方法，该方法允许传入区域性，如果不传入，则使用当前区域性。这将使其具有最大的灵活性和可重复使用性。

/// <summary>
/// Gets the date of the first day of the week for the date.
/// </summary>
/// <param name="date">The date to be used</param>
/// <param name="cultureInfo">If none is provided, the current culture is used</param>
/// <returns>The date of the beggining of the week based on the culture specifed</returns>
public static DateTime StartOfWeek(this DateTime date, CultureInfo cultureInfo=null) =>         
             date.AddDays(-1 * (7 + (date.DayOfWeek - (cultureInfo??CultureInfo.CurrentCulture).DateTimeFormat.FirstDayOfWeek)) % 7).Date;

用法示例：

public static void TestFirstDayOfWeekExtension() {          
        DateTime date = DateTime.Now;
        foreach(System.Globalization.CultureInfo culture in CultureInfo.GetCultures(CultureTypes.UserCustomCulture | CultureTypes.SpecificCultures)) {
            Console.WriteLine($"{culture.EnglishName}: {date.ToShortDateString()} First Day of week: {date.StartOfWeek(culture).ToShortDateString()}");
        }
    }

如果您想要星期六或星期天或一周中的任何一天但不超过当前星期（周六至周日），则可以使用这段代码。

public static DateTime GetDateInCurrentWeek(this DateTime date, DayOfWeek day)
{
    var temp = date;
    var limit = (int)date.DayOfWeek;
    var returnDate = DateTime.MinValue;

    if (date.DayOfWeek == day) return date;

    for (int i = limit; i < 6; i++)
    {
        temp = temp.AddDays(1);

        if (day == temp.DayOfWeek)
        {
            returnDate = temp;
            break;
        }
    }
    if (returnDate == DateTime.MinValue)
    {
        for (int i = limit; i > -1; i++)
        {
            date = date.AddDays(-1);

            if (day == date.DayOfWeek)
            {
                returnDate = date;
                break;
            }
        }
    }
    return returnDate;
}

我们喜欢单线：获取当前区域性的第一天与当前天之间的差，然后从当前天中减去天数

var weekStartDate = DateTime.Now.AddDays(-((int)now.DayOfWeek - (int)DateTimeFormatInfo.CurrentInfo.FirstDayOfWeek));

从“编译此答案”开始，使用以下方法获取星期几的日期：

public static DateTime GetDayOfWeek(this DateTime dt, DayOfWeek day)
{            
    int diff = (7 + (dt.DayOfWeek - DayOfWeek.Monday)) % 7;
    var monday = dt.AddDays(-1 * diff).Date;

    switch (day)
    {
        case DayOfWeek.Tuesday:
            return monday.AddDays(1).Date;

        case DayOfWeek.Wednesday:
            return monday.AddDays(2).Date;

        case DayOfWeek.Thursday:
            return monday.AddDays(3).Date;

        case DayOfWeek.Friday:
            return monday.AddDays(4).Date;

        case DayOfWeek.Saturday:
            return monday.AddDays(5).Date;

        case DayOfWeek.Sunday:
            return monday.AddDays(6).Date;
    }

    return monday;
}

尝试创建一个使用递归的函数。DateTime对象是一个输入，函数返回一个新的DateTime对象，该对象代表一周的开始。

    DateTime WeekBeginning(DateTime input)
    {
        do
        {
            if (input.DayOfWeek.ToString() == "Monday")
                return input;
            else
                return WeekBeginning(input.AddDays(-1));
        } while (input.DayOfWeek.ToString() == "Monday");
    }

通过这种方式计算，您可以选择一周中的哪一天指示新一周的开始（在我选择的示例中为星期一）。

请注意，对于星期一这一天进行此计算将得出当前星期一而不是前一个星期一。

//Replace with whatever input date you want
DateTime inputDate = DateTime.Now;

//For this example, weeks start on Monday
int startOfWeek = (int)DayOfWeek.Monday;

//Calculate the number of days it has been since the start of the week
int daysSinceStartOfWeek = ((int)inputDate.DayOfWeek + 7 - startOfWeek) % 7;

DateTime previousStartOfWeek = inputDate.AddDays(-daysSinceStartOfWeek);