MongoClient client = new MongoClient(host.ConnectionString);

                var server = client.GetServer();

                var dataBase = server.GetDatabase(host.DatabaseName);

                var  collectionClient = dataBase.GetCollection<TestEntity>(host.CollectionName);

               //创建分组结果字典

                Dictionary<string, double> dic_M = new Dictionary<string, double>();

                dic_M["msum"] = 0.0;//分组数据统计值

                //"Sex"，进行分组的字段

                //"function(doc,prev){if(doc.Age>25){ prev.msum++;}}" doc是个实体，获取年龄大于25岁的，进行统计

                //"function(doc){ doc.count=doc.msum;delete doc.msum; }"  把分组数据统计值赋给字典values,并删除分组数据统计值

 var result = collectionClient.Group(null, "Sex", BsonDocument.Create(dic_M), BsonJavaScript.Create("function(doc,prev){if(doc.Age>25){ prev.msum++;}}"), 
                      BsonJavaScript.Create("function(doc){ doc.count=doc.msum;delete doc.msum; }")).ToList();