linq 日期分組統計

 #region         string Condition(DateTime date, string type)         {             if (date == null)             {                 date = DateTime.MinValue;             }             if (type == "day")             {                 return date.ToString("yyyy-M-d");             }             else if (type == "month")             {                 return date.ToString("yyyy-M");             }             return date.Year.ToString();         }         public void xx()         {             List<DateTime> datelist = new List<DateTime>();             var today = DateTime.Now.Date;             for (var i = 0; i < 7;i++ )             {                 datelist.Add(today.AddDays(-i));             }             var oderlist = this.Find().AsEnumerable();             var result = from m in datelist                          join n in oderlist on Condition(m, "day") equals Condition(n.create_time, "day") into c                          from cc in c.DefaultIfEmpty()                          group cc by Condition(m, "day") into g                          select new { g.Key, Total = (g.FirstOrDefault()==null?0:g.Count())};             var ccc = result.ToList();         }         #endregion