public IList <T> GetBy <T>(string appid, string id)
        {
            var mongodb    = MongoDBHelper.GetMongoDB();
            var collection = mongodb.GetCollection <T>(_colname);

            FilterDefinition <T> wheres = Builders <T> .Filter.Eq("AppId", appid);

            if (!string.IsNullOrWhiteSpace(id))
            {
                wheres = wheres & Builders <T> .Filter.Eq("Id", id);
            }

            return(collection.Find <T>(wheres).ToList <T>());
        }