DomainWithConstraints.cs

class CreateTheme
{
	public int Id;
	public int AccountId;
	public string Name;
	public Optional<string> Description;
}

static void Handle(IUnitOfWork uow, CreateTheme cmd)
{
	var account = uow.Load<Account>(cmd.AccountId);
	uow.Register(account.Create(cmd.Name, cmd.Description));
}


class Theme
{
	public readonly int Id;
	public readonly int AccountId;
	public readonly string Name;
	public readonly string Description;

	public Theme(int id, int accountId, string name, string description)
	{
		Guard.Against(id < 1, ()=> Argument.Invalid("id should be greater than 0"));
		Guard.AgainstNullOrEmpty(()=> name);

		Id = id;
		AccountId = accountId;
		Name = name;
		Description = description;
	}
}

class Account
{
	const int Max_Themes_Per_Account = 10;

	readonly IList<Theme> themes = new List<Theme>();

	public Theme Create(string name, string description)
	{
		if (themes.Count + 1 > Max_Themes_Per_Account)
			throw MaxThemePerAccountLimitReached.Create(this, name);

		return new Theme(themes.Count + 1, this.Id, name, description);
	}
}