C#获取周的第一天、最后一天、月第一天和最后一天

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public class DateTimeTool
{
/// <summary>
/// 获取指定日期所在周的第一天，星期天为第一天
/// </summary>
/// <param name="dateTime"></param>
/// <returns></returns>
public static DateTime GetDateTimeWeekFirstDaySun(DateTime dateTime)
{
DateTime firstWeekDay = DateTime.Now;

try
{
//得到是星期几，然后从当前日期减去相应天数
int weeknow = Convert.ToInt32(dateTime.DayOfWeek);

int daydiff = (-1) * weeknow;

firstWeekDay = dateTime.AddDays(daydiff);
}
catch { }

return firstWeekDay;
}

/// <summary>
/// 获取指定日期所在周的第一天，星期一为第一天
/// </summary>
/// <param name="dateTime"></param>
/// <returns></returns>
public static DateTime GetDateTimeWeekFirstDayMon(DateTime dateTime)
{
DateTime firstWeekDay = DateTime.Now;

try
{
int weeknow = Convert.ToInt32(dateTime.DayOfWeek);

//星期一为第一天，weeknow等于0时，要向前推6天。
weeknow = (weeknow == 0 ? (7 - 1) : (weeknow - 1));

int daydiff = (-1) * weeknow;

firstWeekDay = dateTime.AddDays(daydiff);
}
catch { }

return firstWeekDay;
}

/// <summary>
/// 获取指定日期所在周的最后一天，星期六为最后一天
/// </summary>
/// <param name="dateTime"></param>
/// <returns></returns>
public static DateTime GetDateTimeWeekLastDaySat(DateTime dateTime)
{
DateTime lastWeekDay = DateTime.Now;

try
{
int weeknow = Convert.ToInt32(dateTime.DayOfWeek);

int daydiff = (7 - weeknow) - 1;

lastWeekDay = dateTime.AddDays(daydiff);

}catch { }

return lastWeekDay;
}

/// <summary>
/// 获取指定日期所在周的最后一天，星期天为最后一天
/// </summary>
/// <param name="dateTime"></param>
/// <returns></returns>

public static DateTime GetDateTimeWeekLastDaySun(DateTime dateTime)
{
DateTime lastWeekDay = DateTime.Now;

try
{
int weeknow = Convert.ToInt32(dateTime.DayOfWeek);

weeknow = (weeknow == 0 ? 7 : weeknow);

int daydiff = (7 - weeknow);

lastWeekDay = dateTime.AddDays(daydiff);
}
catch { }

return lastWeekDay;
}

/// <summary>
/// 获取指定日期的月份第一天
/// </summary>
/// <param name="dateTime"></param>
/// <returns></returns>
public static DateTime GetDateTimeMonthFirstDay(DateTime dateTime)
{
if (dateTime == null)
{
dateTime = DateTime.Now;
}

return new DateTime(dateTime.Year, dateTime.Month, 1);
}

/// <summary>
/// 获取指定月份最后一天
/// </summary>
/// <param name="dateTime"></param>
/// <returns></returns>
public static DateTime GetDateTimeMonthLastDay(DateTime dateTime)
{
int day = DateTime.DaysInMonth(dateTime.Year, dateTime.Month);

return new DateTime(dateTime.Year, dateTime.Month, day);
}
}