方法一： //最有含量的一种

int days = System.Threading.Thread.CurrentThread.CurrentUICulture.Calendar.GetDaysInMonth(DateTime.Now.Year ,DateTime.Now.Month); 

方法二：//最奇怪的一种

DateTime dtNow = DateTime.Today;     int days = dtNow .AddDays(1 - dtNow .Day).AddMonths(1).AddDays(-1).Day;

方法三： //最常规的一种

DateTime dtNow = DateTime.Now;     int days = DateTime.DaysInMonth(dtNow.Year ,dtNow.Month);