下面的方法应该返回所需的DateTime。周日为true，周一为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);
}

使用扩展方法:

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;
    }
}

可以这样使用:

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

通过这种方式计算，您可以选择一周中的哪一天表示新一周的开始(在示例中，我选择了星期一)。

请注意，对星期一进行这个计算将得到当前的星期一，而不是以前的星期一。

//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);

这将给你前一个星期天(我想):

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

我们喜欢简单的语句:获取当前文化中每周的第一天与当前日期之间的差值，然后从当前日期减去天数:

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

c#中的模数对-1 mod 7(它应该是6，但c#返回-1)效果很差 所以…一个“一行程序”的解决方案是这样的:

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