c#获取某月的第一天和某月的最后一天

/// <summary>
/// 取得某月的第一天
/// </summary>
/// <param name="datetime">要取得月份第一天的时间</param>
/// <returns></returns>
private DateTime FirstDayOfMonth(DateTime datetime)
{
return datetime.AddDays(1 - datetime.Day);
}

/**//// <summary>
/// 取得某月的最后一天
/// </summary>
/// <param name="datetime">要取得月份最后一天的时间</param>
/// <returns></returns>
private DateTime LastDayOfMonth(DateTime datetime)
{
return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);
}

/**//// <summary>
/// 取得上个月第一天
/// </summary>
/// <param name="datetime">要取得上个月第一天的当前时间</param>
/// <returns></returns>
private DateTime FirstDayOfPreviousMonth(DateTime datetime)
{
return datetime.AddDays(1 - datetime.Day).AddMonths(-1);
}

/**//// <summary>
/// 取得上个月的最后一天
/// </summary>
/// <param name="datetime">要取得上个月最后一天的当前时间</param>
/// <returns></returns>
private DateTime LastDayOfPrdviousMonth(DateTime datetime)
{
return datetime.AddDays(1 - datetime.Day).AddDays(-1);
}