// We only accept payments matching the Installment Amount.
public void MakePayment(decimal amount, Guid installmentId)
{
Installment installment = Installments.First(i => i.Id == installmentId);
if (amount != installment.Amount)
{
throw new ArgumentException();
}
installment.SetPaid(Id, DateTime.Now);
}
public Installment FirstInstallment()
{
return(Installments.First());
}