var group = new BsonDocument {

                {"_id",new BsonDocument

                {

                    {"stationID","$stationID"},

                    {"stationName","$stationName" }

                }

        },

                //{"stationID",new BsonDocument{ { "stationID", "$stationID" } } },

                {"stationID",new BsonDocument("$last","$stationID")},

                { "TimeData",new BsonDocument("$max","$TimeData")},

                {"eValue" ,new BsonDocument("$last","$eValue")}

            };

var collection = MoDataBase.GetCollection<BsonDocument>(tablename);

 var list = collection.Aggregate().Group(group).ToListAsync().Result;

            double value = 0.0;

            foreach (BsonDocument bsondoc in list)

            {

                double evalue = double.TryParse(bsondoc.GetElement("eValue").Value.ToString(), out value) ? value : ;

                string stationid = bsondoc.GetElement("stationID").Value.ToString();

/// <summary>

        /// 获取数据库中每条数据对应的时间放到图表的横坐标上

        /// </summary>

        /// <param name="StatioID">监测站ID</param>

        /// <param name="PullutionID">指标类型</param>

        /// <returns></returns>

        public static List<string> GetChartData(string StatioID, string PullutionID,string StartTime,string EndTime)

        {

            List<string> XData = new List<string>();

            if (ConnectionString == null) OnCreateDB();

            MoDataBase = MoClient.GetDatabase("SSMonitor");

            BsonDocument bsonDoc = new BsonDocument();

            bsonDoc.Add("TimeData", new BsonDocument() { { "$gte", DateTimeToInt(StartTime) }, { "$lte", DateTimeToInt(EndTime) } });

            var collection = MoDataBase.GetCollection<BsonDocument>(string.Format("{0}_{1}_Day", StatioID, PullutionID));

            var sort = Builders<BsonDocument>.Sort.Ascending("TimeData");

            var list = collection.Find(bsonDoc).Sort(sort).ToList();

            foreach (BsonDocument bsondoc in list)

            {

                string time = bsondoc.GetElement("MonitorTime").Value.ToString();

                XData.Add(time);

            }

            return XData;

        }