一、获取上个月第一天

 

private DateTime FirstDayOfPreviousMonth(DateTime datetime)
{
    return datetime.AddDays(1 - datetime.Day).AddMonths(-1);
}

二、获取上个月的最后一天

 

private DateTime LastDayOfPrdviousMonth(DateTime datetime)
{
    return datetime.AddDays(1 - datetime.Day).AddDays(-1);
}

参考资料：C#中如何获取上个月第一天和最后一天   http://www.studyofnet.com/news/1183.html