Program.cs

// In BusinessLayer.dll
using System;

public class User
{
	public User(string username)
	{
		Id = Guid.NewGuid();
		CreatedOn = DateTime.UtcNow;
		Username = username;
	}

	public Guid Id { get; private set; }

	public DateTime CreatedOn { get; private set; }

	public string Username { get; private set; }

	public void Rename(string username)
	{
		// ...
	}
}

public interface IUsersRepository
{
	User Find(Guid id);

	void Save(User user);
}

// In DataLayer.dll
using System;
using MongoDB.Bson.Serialization;

public class UserMap : BsonClassMap<User>
{
	public UserMap()
	{
		Id(x => x.Id);
		MapProperty(x => x.CreatedOn);
		MapProperty(x => x.Username);
	}
}

public class MongoDbUsersRepository : IUsersRepository
{
	public User Find(Guid id)
	{
		// ...
	}

	public void Save(User user)
	{
		// ...
	}
}

// In Application.exe
public class Program
{
	public static void Main()
	{
		// TODO: Use IoC/DI to find implementations...
		var usersRepository = new MongoDbUsersRepository();

		var user = new User("Kim^J");

		usersRepository.Save(user);

		// ...
	}
}