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1、获取某日期所在周的第一天

public DateOnly GetFirstDayOfWeek(DateTime dateTime)        {            var culture = CultureInfo.CurrentCulture;            var differenceDays = dateTime.DayOfWeek - culture.DateTimeFormat.FirstDayOfWeek;            if (differenceDays < 0)            {                // 如果差值小于零，则将该差值加 7                differenceDays += 7;            }            // 给指定日期减去给定差值            dateTime = dateTime.AddDays(-differenceDays).Date