How can i shoud handle the inyection of services domain

I'm building a finnancial application where i have two ways to calculate the balance of the credit so try to follow a design based on Domain Driven Design and i feel some confused respect how supose that i have to inyect the balance calculator in the credit entity. i try to insert a interface in credit called IBalanceCalculator then when i call method CalculateBalanceAtDate in the credit this determine what instance use

this is the example

credit {
  protected  IBalanceCalcultar _calculator;
  ....
    private void _InitializeBalanceCalculator()
    {
        if (_balanceCalculator == null)
            switch (InterestType)
            {
                case EInterestType.OutstandingBalance:
                    _balanceCalculator = new OutstandingBalanceService();
                    break;
                case EInterestType.GlobalBalance:
                    _balanceCalculator = new GlobalBalance();
                    break;
                default:
                    throw new Exception();
            }
    }
    public void CalculateBalanceAtDate(DateTime date, bool moratory)
    {
        _InitializeBalanceCalculator();
        _balanceCalculator.GetBalance(this, date);
    }
}

but i feel that this way is not so good

someone can clarify me is correct or if exists a better way