如何在MongoDB C#中构建条件查询

我在使用 C# MongoDB 驱动程序构建条件查询时遇到问题。每当我运行下面的代码时，我都会得到一个空列表。任何帮助将不胜感激。

这是我的函数

 public async void searchBook()
 {
        Book book = new Book();
        IMongoDatabase mdb = MongoDBConnectionManager.ConnectToMongoDB();
        var query = new BsonDocument();            
        if (queryString.ContainsKey("Title"))
        {
            query.Add("Title", queryString["Title"]);
        }
        if (queryString.ContainsKey("ISBN"))
        {
            query.Add("Isbn", queryString["ISBN"]);
        }
        if (queryString.ContainsKey("Author"))
        {
            query.Add("Author", queryString["Author"]);
        }
        if (queryString.ContainsKey("Publisher"))
        {
            query.Add("Publisher", queryString["Publisher"]);
        }
        var collection = mdb.GetCollection<Book>("Book");
        var sort = Builders<Book>.Sort.Ascending("Title");
        if(query.ElementCount > 0)
        {
            var list = await collection.Find(query).Sort(sort).ToListAsync();
            dt = ConvertToDataTable(list);
            BindGridView(dt);
        }
        else
        {
            var list = await collection.Find(Builders<Book>.Filter.Empty).Sort(sort).ToListAsync();
            dt = ConvertToDataTable(list);
            BindGridView(dt);
        }                                
    }

        var query = collection.AsQueryable();
        if (!string.IsNullOrEmpty(entity.Name))
            query = query.Where(p => p.Name.Contains(entity.Name));
        if (!string.IsNullOrEmpty(entity.Description))
            query = query.Where(p => p.Description.Contains(entity.Description));
        var YourList=query.ToList();