C.NET从当前日期开始获取上个月的最后一天

var now = DateTime.Now;    
var firstDayCurrentMonth = new DateTime(now.Year, now.Month, 1);

var lastDayLastMonth = firstDayCurrentMonth.AddDays(-1);

DateTime now = DateTime.Now;
DateTime lastDayOfLastMonth = now.Date.AddDays(-now.Day);

试试 DateTime.DaysInMonth(int year, int month) method

下面是一个例子:

DateTime oneMonthAgo = DateTime.Now.AddMonths(-1);
int days = DateTime.DaysInMonth(oneMonthAgo.Year, oneMonthAgo.Month);