更新MongoDB集合中的一个条目

我不知道如何更新我的MongoDB集合中的一个条目。我已经看了c#驱动程序文档，我认为我已经非常密切地遵循了它们。

但是，我传递给Update方法的一个(或者两个?)参数是无效的。有人能告诉我我做错了什么吗?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using MongoDB.Bson;
using MongoDB.Bson.Serialization.Attributes;
using MongoDB.Driver;
using MongoDB.Driver.Linq;

namespace Csharp_Linq
{
    class Program
    {
        public class Book
        {
            // Fields
            public string title { get; set; }
            public string author { get; set; }
            public ObjectId id { get; set; }
            // Constructors
            public Book()
            {
                this.title = "some title";
                this.author = "some author";
            }
            public Book(string title, string author)
            {
                this.title = title;
                this.author = author;
            }
        }
        static void Main(string[] args)
        {
            // Connect to the server
            string connectionString = "mongodb://localhost";
            MongoClient client = new MongoClient(connectionString);
            MongoServer server = client.GetServer();
            // Get the database then the collection
            MongoDatabase database = server.GetDatabase("tutorial");
            MongoCollection collection = database.GetCollection("books");
            // Query the collection
            int count =
                (from book in collection.AsQueryable<Book>()
                 select book)
                .Count();
            string numBooks = String.Format("This collection has {0} books.", count);
            Console.WriteLine(numBooks);
            var query =
                from book in collection.AsQueryable<Book>()
                where book.author == "Ernest Hemingway"
                select book;
            foreach (var book in query)
            {
                string bookInfo = String.Format("{0} by {1}", book.title, book.author);
                Console.WriteLine(bookInfo);
            }
            // Insert new books
            Book scaryBook = new Book("Dr. Sleep", "Stephen King");
            Book[] batch = 
            {
                new Book(),
                scaryBook
            };
            collection.InsertBatch(batch);
            // Update default book
            var query2 =
                from book in collection.AsQueryable<Book>()
                where book.title == "some title" && book.author == "some author"
                select book;
            var update = new UpdateDocument {
                { "$set", new BsonDocument("title", "War and Peace") }
            };
            BsonDocument updatedBook = collection.Update(query2, update);
            Console.ReadLine();
        }
    }
}

我有点惊讶，更新方法实际上返回一个BsonDocument。为什么会这样呢?

前面我尝试使用Update对象，如示例所示:

MongoCollection<BsonDocument> books;
var query = Query.And(
    Query.EQ("author", "Kurt Vonnegut"),
    Query.EQ("title", "Cats Craddle")
);
var update = Update.Set("title", "Cat's Cradle");
BsonDocument updatedBook = books.Update(query, update);

这个对象还存在吗?每当我在Visual Studio中输入它时，我都会得到一个错误，说对象不在命名空间中。