查找日历的第一天
我想要做的是创建一个简单的日历，我想找到特定月份的第一周的第一天。 我的日历是星期一 – >星期日日历，下面的代码可以正常工作，但是你可以看到它并不是那么好。 任何人都可以更好地了解如何在日历中获得第一个日期。
var now = new DateTime(Year, Month, 1); now = now.AddDays(1-(int)now.DayOfWeek); now = now.Day > 15 ? now : now.AddDays(-7); 日历最终将如下所示：
| | ------------------------------------ | Mo | Tu | We | Th | Fr | Sa | Su | |[27]| 28 | 29 | 30 | 31 | 01 | 02 | | 03 | 04 | 05 | 06 | 07 | 08 | 09 | | .. | .. | .. | .. | .. | .. | .. | | .. | .. | .. | .. | .. | .. | .. | | 31 | 01 | 02 | 03 | 04 | 05 | 06 |
在这个“形象”中，这是我试图找到的[27]日期。
解决方案（发现我更好/更清洁循环然后计算）：
public DateTime FirstDay() { var date = new DateTime(Date.Year, Date.Month, 1); while (true) { if (date.DayOfWeek == DayOfWeek.Monday) return date; date = date.AddDays(-1); } return date; } public DateTime LastDay() { var date = new DateTime(Date.Year, Date.Month, DateTime.DaysInMonth(Date.Year, Date.Month)); while (true) { if (date.DayOfWeek == DayOfWeek.Sunday) return date; date = date.AddDays(1); } return date; }
/ BR Andreas
我会这样做的。 这很容易理解：
var firstDayOfMonth = new DateTime(year, month, 1); DateTime startOfCalendar = FirstDayOfWeekOnOrBefore( firstDayOfMonth, DayOfWeek.Monday ); public static DateTime FirstDayOfWeekOnOrBefore( DateTime date, DayOfWeek dayOfWeek ) { while(date.DayOfWeek != dayOfWeek) { date = date.AddDays(-1); } return date; }
此外，如果您想要将日历更改为星期一以外的其他内容，那么现在就可以了。 使用模运算的解决方案不可维护。
您可以使用模数来计算没有条件语句的填充天数：
DateTime firstOfMonth=new DateTime(year,month,1); var weekDay=firstOfMonth.DayOfWeek; int fillerDays=((int)weekDay+6)%7; DateTime firstDayInCalendar=firstOfMonth.AddDays(-fillerDays);
假设第一天你指的是星期一，你可以试试这个
DateTime dt = new DateTime(2011, 2, 2); Console.WriteLine(dt.AddDays((8 - (int)dt.DayOfWeek) % 7));
我不喜欢while循环，因为它们与LINQ一起使用时很昂贵
希望其他人可以重复使用此代码:(如果你在美国，那么只需删除[+ 6）％7）]两行）
/// /// Expands the month. /// | < | Jan 2011 | > | /// ------------------------------------ /// | Mo | Tu | We | Th | Fr | Sa | Su | /// |[27]| 28 | 29 | 30 | 31 | 01 | 02 | /// | 03 | 04 | 05 | 06 | 07 | 08 | 09 | /// | .. | .. | .. | .. | .. | .. | .. | /// | .. | .. | .. | .. | .. | .. | .. | /// | 31 | 01 | 02 | 03 | 04 | 05 | 06 | /// /// Some day in the month of interest, the start date is updated to become the date of firstDayInCalendar /// The number of days to show. This value is either (28, 35 or 42) public static int ExpandMonth(ref DateTime start) { DateTime first = new DateTime(start.Year, start.Month, 1); DateTime last = new DateTime(start.Year, start.Month, DateTime.DaysInMonth(start.Year, start.Month)); start = first.AddDays(-((int)first.DayOfWeek + 6) % 7); last = last.AddDays(7 - ((int)last.DayOfWeek + 6) % 7); return last.Subtract(start).Days; }
//托马斯
我发现自己经常需要这样做，所以我创建了以下扩展方法。
public static DateTime FirstDateOfCalendarMonth(this DateTime dt, DayOfWeek firstDayOfWeek = DayOfWeek.Sunday) { dt = new DateTime(dt.Year, dt.Month, 1); while (dt.DayOfWeek != firstDayOfWeek){ dt = dt.AddDays(-1); } return dt; }
像这样使用它
var firstCalDate = DateTime.Now.FirstDateOfCalendarMonth();
它默认为星期日作为第一个DayOfWeek，但你可以将它传递给你喜欢的DayOfWeek，如下所示：
var firstCalDate = DateTime.Now.FirstDateOfCalendarMonth(DayOfWeek.Monday);
DateTime start_date = Cal_start_date.SelectedDate; DateTime end_date = Cal_end_date.SelectedDate; Dictionary dic_week_day = new Dictionary(); dic_week_day["Sunday"] = 1; dic_week_day["Monday"] = 2; dic_week_day["Tuesday"] = 3; dic_week_day["Wednesday"] = 4; dic_week_day["Thursday"] = 5; dic_week_day["Friday"] = 6; dic_week_day["Saturday"] = 7; DateTime first_day = start_date.AddDays(1 - start_date.Day); int selected_day = dic_week_day[Drp_day_of_week.SelectedValue.ToString()]; string str_html = ""; for (DateTime i = first_day; i <= end_date; i = i.AddMonths(1)) { int day_of_week = dic_week_day[i.DayOfWeek.ToString()]; DateTime temp_date; if (day_of_week > selected_day) { temp_date = i.AddDays((7 - day_of_week) + selected_day); } else { temp_date = i.AddDays(selected_day - day_of_week); } DateTime last_day_of_month = (temp_date.AddMonths(1)).AddDays(-temp_date.Day); if (Drp_occurrence.SelectedValue.ToString() == "odd") { if (start_date <= temp_date && temp_date <= end_date && temp_date <= last_day_of_month) { str_html += "
" + temp_date.ToString(); } DateTime res_date = temp_date.AddDays(14); if (start_date <= res_date && res_date <= end_date && res_date <= last_day_of_month) { str_html += " , " + res_date.ToString(); } res_date = temp_date.AddDays(28); if (start_date <= res_date && res_date <= end_date && res_date <= last_day_of_month) { str_html += " , " + res_date.ToString(); } } else if (Drp_occurrence.SelectedValue.ToString() == "even") { DateTime res_date = temp_date.AddDays(7); if (start_date <= res_date && res_date <= end_date && res_date <= last_day_of_month) { str_html += "
" + res_date.ToString(); } res_date = temp_date.AddDays(21); if (start_date <= res_date && res_date <= end_date && res_date <= last_day_of_month) { str_html += " , " + res_date.ToString(); } } else { int occurrence = Int32.Parse(Drp_occurrence.SelectedValue.ToString()); DateTime res_date = temp_date.AddDays((occurrence - 1) * 7); if (start_date <= res_date && res_date <= end_date && res_date <= last_day_of_month) { str_html += "
" + res_date.ToString(); } } } Div_result.InnerHtml = str_html;
更新：以下代码有错误！ 使用模运算，以补偿环境，.NET在星期天开始一周！ 请参阅此处的其他解决方案（没有额外function的解决方案）。
如果您需要找到“27”（即显示一个月的第一天），只需使用：
DateTime monthStart = new DateTime(year, month, 1); DateTime monthDisplayStart = monthStart.AddDays(-((int)monthStart.DayOfWeek - 1));