Mongodb在CSharp里实现Aggregate实例

/// <summary>
    /// 返回UI消息树
    /// </summary>
    /// <returns></returns>
    public static string GetMongoLog(DateTime? fromDate, DateTime? toDate, int page = 1)
    {
      string from = DateTime.Now.Date.ToString("yyyy-MM-dd");
      string to = DateTime.Now.Date.AddDays(1).ToString("yyyy-MM-dd");
      if (fromDate.HasValue)
      {
        from = fromDate.Value.ToString("yyyy-MM-dd");

      }
      if (toDate.HasValue)
      {
        to = toDate.Value.ToString("yyyy-MM-dd");
      }
      var stages = new List<IPipelineStageDefinition>();
      stages.Add(new JsonPipelineStageDefinition<BsonDocument, BsonDocument>("{$match:{AddTime:{$gt:ISODate('" + from + "'),$lt:ISODate('" + to + "')}}}"));
      stages.Add(new JsonPipelineStageDefinition<BsonDocument, BsonDocument>("{$group:{_id: \"$RootId\", count: {$sum: 1}}}"));
      stages.Add(new JsonPipelineStageDefinition<BsonDocument, BsonDocument>("{$skip:" + page * 5 + "}"));
      stages.Add(new JsonPipelineStageDefinition<BsonDocument, BsonDocument>("{$limit:5}"));
      var pipeline = new PipelineStagePipelineDefinition<BsonDocument, BsonDocument>(stages);
      var result = NoSql.MongodbManager<LoggerContext>.Collection.Aggregate(pipeline);
      StringBuilder str = new StringBuilder();

      str.Append("<ol class='treeMsg'>");
      foreach (var item in result.ToList())
      {
        var timer = new List<DateTime>();
        var old = NoSql.MongodbManager<LoggerContext>.Instance.Find(i => i.RootId == item.Values.ToArray()[0].ToString() && i.ParentId == null).FirstOrDefault();
        timer.Add(old.AddTime);
        str.Append("<li style='margin:5px;border:1px dashed #aaa'>");
        str.AppendFormat("<span style='color:red;'>{0}</span><span style='color:green'>{1}</span><span>{2}</span>"
          , old.Url
          , old.MessageBody
          , old.AddTime);
        MsgTree(str, old.ChildId, timer);
        str.AppendFormat("<p><em>本次请求用时{0}毫秒（{1}秒）<em></p>"
          , (timer.Max() - timer.Min()).TotalMilliseconds
          , (timer.Max() - timer.Min()).TotalSeconds);
        str.Append("</li>");
      }
      str.Append("</ol>");
      return str.ToString();
    }