Как получить часть документа в mongodb?

Разобрался.

class User
{
    [BsonId]
    public ObjectId Id { get; set; }
    [BsonElement("userId")]
    [Required]
    public long UserId { get; set; }
    [BsonElement("firstName")]
    public string FirstName { get; set; }
    [BsonElement("lastName")]
    public string LastName { get; set; }
}
class UserLastName
{
    [BsonId]
    public ObjectId Id { get; set; }
    [BsonElement("lastName")]
    public string LastName{ get; set; }
}

var client = new MongoClient("mongodb://user:pass@server");
        var db = client.GetDatabase("collection");
        var collection = db.GetCollection<User>("users");
        var projection = Builders<User>.Projection.Include("lastName");
        var result = collection.Find(o => o.UserId == userId).Project<UserLastName>(projection).ToList();
        Console.WriteLine(result[0].LastName);

Использовал VS2017 и MongoDB.driver версии 2.4.4.