private void AddNewAccount(Account newAccount)
 {
     foreach (Account account in this.accounts)
     {
         if (account.Customer.ID == newAccount.Customer.ID)
         {
             throw new InvalidOperationException("Cannot have two customers with the same ID!");
         }
     }
     this.accounts.Add(newAccount);
 }
 public void RemoveAccount(Account account)
 {
     this.accounts.Remove(account);
 }
 public void AddAccount(Account newAccount)
 {
     AddNewAccount(newAccount);
 }