c# 取当月第一天至下月第一天

DateTime FirstDay = DateTime.Now.AddDays(-DateTime.Now.Day + 1).AddHours(-DateTime.Now.Hour).AddMinutes(-DateTime.Now.Minute).AddSeconds(-DateTime.Now.Second);
DateTime LastDay = DateTime.Now.AddMonths(1).AddDays(-DateTime.Now.AddMonths(1).AddDays(-1).Day).AddHours(-DateTime.Now.Hour).AddMinutes(-DateTime.Now.Minute).AddSeconds(-DateTime.Now.Second);

 new:

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