public class Address
    {
        public int Zip       { get; set; }
        public string City   { get; set; }
        public string Street { get; set; }
        public int HomeNr { get; set; }
    }

    public class Client
    {
        [BsonId]
        public ObjectId _id { get; set; }
        public int AccountNr { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public List<Address> Addresses { get; set; }
        [BsonConstructor]
        public Client()
        {
            Addresses=new List<Address>();
        }
 
    }

        public void InsertCustomer(Client client )
        {
            MongoCollection<Client> document = myDatabase.GetCollection<Client>("Client");
            BsonDocument Client = new BsonDocument
            {
                {"AccountNr",client.AccountNr},
                {"FirstName",client.FirstName},
                {"LastName",client.LastName},
            };
            document.Insert(Client);
        }