Programming pattern / architectural question

Question

I am currently working on a project where I have a BankAccount entity for some other entity.

Each bank account as a reference to a bank entity, an account number and opt

Answer 1

You can just use a delegate to validate, you don't need to pass a whole interface, who ever wants to set it has to have a validator.

public delegate bool Validation(IBAN iban);
void SetIBAN(IBAN iban, Validation isValid){ 
  if(!isValid(iban)) throw new ArgumentException();
...}