返回日期的月初与月末

string yyyymm="201111";

string firstdate = yyyymm.Substring(0, 4) + "/" + yyyymm.Substring(4, 2) + "/" + "01";  //2011/11/01
string lastdate = yyyymm.Substring(0, 4) + "/" + yyyymm.Substring(4, 2) + "/" + DateTime.DaysInMonth(int.Parse(yyyymm.Substring(0, 4)), int.Parse(yyyymm.Substring(4, 2)));  //2011/11/30