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);

解决方案

回答

这将为我们提供前一个星期日(我认为)：

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

回答

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);
}

回答

这可能有点hack，但是我们可以将.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)，并使用该值来确定一周的上一个开始。

它似乎在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);

回答

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);

回答

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;

回答

这将使我们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);

这使我们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);

回答

我们可以使用出色的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(请参见属性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：仔细看这个问题，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();

虽然值得注意的是，如果我们在星期一要求上一个星期一，它将给我们7天的时间。但是，如果我们使用BeginningOfWeek，也是如此，这似乎是一个错误:(。

回答

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));
    }
}

回答

使用Fluent DateTime：

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

回答

public static System.DateTime getstartweek()
{
    System.DateTime dt = System.DateTime.Now;
    System.DayOfWeek dmon = System.DayOfWeek.Monday;
    int span = dt.DayOfWeek - dmon;
    dt = dt.AddDays(-span);
    return dt;
}

回答

感谢示例。我需要始终在一周的第一天使用" CurrentCulture"，对于数组，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
    }
}

回答

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;

    }

我已经在博客上发布了用于计算周，月，季度和年的开始/结束的完整代码
扎米尔斯博客