C# 處理DateTime算法，取某月第1天及最后一天

 

代碼如下所示：

/// <summary>  
/// 取得某月的第一天  
/// </summary>  
/// <param name="datetime">要取得月份第一天的時間</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時間</param>  
/// <returns></returns>  
private DateTime LastDayOfMonth(DateTime datetime)  
  
    return datetime.AddDays(1 - datetime.Day).AddMonths(1).AddDays(-1);  
}  
  
/// <summary>  
/// 取得上個月第一天  
/// </summary>  
/// <param name="datetime">要取得上個月第一天的當前時間</param>  
/// <returns></returns>  
private DateTime FirstDayOfPreviousMonth(DateTime datetime)  
{  
    return datetime.AddDays(1 - datetime.Day).AddMonths(-1);  
}  
  
/// <summary>  
/// 取得上個月的最后一天  
/// </summary>  
/// <param name="datetime">要取得上個月最后一天的當前時間</param>  
/// <returns></returns>  
private DateTime LastDayOfPrdviousMonth(DateTime datetime)  
{  
    return datetime.AddDays(1 - datetime.Day).AddDays(-1);  
}