连接条件在linq中给出错误

Question

我在linq下面使用但是它在第一个“join”关键字

上给出了错误

(from st in queryResult.ToList()
 join rf in rainfall.ToList() on Convert.ToInt32(st.StationID) equals Convert.ToInt32(rf.StationID)
 join wl in water on 
 new { StationID = Convert.ToInt32(st.StationID), DataRecieved = rf.DataRecieved } 
 equals
 new { StationID = Convert.ToInt32(wl.StationId), DataRecieved = wl.DataRecieved } into gj
 from subpet in gj.DefaultEmpty()
                                     select new
                                     {
                                     Stationname = st.Stationname,
                                     Stationid = st.stationid,
                                     State = st.state,
                                     Datarecieved = rf.datarecieved,
                                     dailyrainfall = rf.dailyrainfall,
                                     cumlativerainfall = rf.cumrainfall,
                                     waterlevel1 = subpet.waterlevel1

                                     })

给出错误“不包含”DefaultEmpty“的定义，并且没有接受第一个参数的扩展方法”DefaultEmpty“。