1.cs

    public class PublishedAccount
    {
        readonly DateTime publishedDate;
        readonly Maybe<DateTime> publishedDateEnd;
        readonly AccountTypeStandard standardAccount;
        readonly AccountTypeNonstandard nonstandardAccount;

        public PublishedAccount(DateTime publishedDate, Maybe<DateTime> publishedDateEnd, AccountTypeStandard standardAccount, AccountTypeNonstandard nonstandardAccount)
        {
            this.publishedDate = publishedDate;
            this.publishedDateEnd = publishedDateEnd;
            this.standardAccount = standardAccount;
            this.nonstandardAccount = nonstandardAccount;
        }

        public static PublishedAccount Parse(ZverejnenyUcetType ucet)
        {
            return new PublishedAccount(
                    ucet.datumZverejneni,
                    ucet.datumZverejneniUkonceni.ToMaybe(),
                    ucet.Item.GetType() == typeof(StandardniUcetType)
                        ? AccountTypeStandard.Parse((StandardniUcetType)ucet.Item)
                        : null,
                    ucet.Item.GetType() == typeof(NestandardniUcetType)
                        ? AccountTypeNonstandard.Parse((NestandardniUcetType)ucet.Item)
                        : null
                );
        }

        public XElement ToXElement()
        {
            return new XElement(Ns.Response + "PublishedAccount",
                new XElement(Ns.Response + "PublishDate", publishedDate),
                publishedDateEnd.ToXElement(Ns.Response + "PublishDateEnd"),
                standardAccount != null ? standardAccount.ToXElement() : null,
                nonstandardAccount != null ? nonstandardAccount.ToXElement() : null);
        }
    }

2.cs

    public class PublishedAccount
    {
        readonly DateTime publishedDate;
        readonly Maybe<DateTime> publishedDateEnd;
        readonly AccountType account;

        public PublishedAccount(DateTime publishedDate, Maybe<DateTime> publishedDateEnd, AccountType account)
        {
            this.publishedDate = publishedDate;
            this.publishedDateEnd = publishedDateEnd;
            this.account = account;
        }

        public static PublishedAccount Parse(ZverejnenyUcetType ucet)
        {
            return new PublishedAccount(
                ucet.datumZverejneni,
                ucet.datumZverejneniUkonceni.ToMaybe(),
                AccountType.Parse(ucet.Item));
        }

        public XElement ToXElement()
        {
            return new XElement(Ns.Response + "PublishedAccount",
                new XElement(Ns.Response + "PublishDate", publishedDate),
                publishedDateEnd.ToXElement(Ns.Response + "PublishDateEnd"),
                account.ToXElement());
        }
    }

    public class AccountType
    {
        readonly AccountTypeStandard standardAccount;
        readonly AccountTypeNonstandard nonstandardAccount;

        public AccountType(AccountTypeStandard standardAccount)
        {
            this.standardAccount = standardAccount;
        }

        public AccountType(AccountTypeNonstandard nonstandardAccount)
        {
            this.nonstandardAccount = nonstandardAccount;
        }

        public static AccountType Parse(object account)
        {
            var standard = account as StandardniUcetType;
            if (standard != null)
                return new AccountType(AccountTypeStandard.Parse(standard));

            var nonstandard = account as NestandardniUcetType;
            return new AccountType(AccountTypeNonstandard.Parse(nonstandard));
        }

        public XElement ToXElement()
        {
            return standardAccount != null ? standardAccount.ToXElement() : nonstandardAccount.ToXElement();
        }
    }

3.cs

    public abstract class AccountType
    {
        public static AccountType Parse(object account)
        {
            var standard = account as StandardniUcetType;
            if (standard != null)
                return StandardAccount.Parse(standard);

            var nonstandard = account as NestandardniUcetType;
            return NonstandardAccount.Parse(nonstandard);
        }

        public abstract XElement ToXElement();

        class StandardAccount : AccountType
        {
            readonly string accountPrefix;
            readonly string accountNumber;
            readonly string bankCode;

            StandardAccount(string accountPrefix, string accountNumber, string bankCode)
            {
                this.accountPrefix = accountPrefix;
                this.accountNumber = accountNumber;
                this.bankCode = bankCode;
            }

            public static StandardAccount Parse(StandardniUcetType ucet)
            {
                return new StandardAccount(ucet.predcisli, ucet.cislo, ucet.kodBanky);
            }

            public override XElement ToXElement()
            {
                return new XElement(Ns.Response + "AccountTypeStandard",
                    new XElement(Ns.Response + "AccountPrefix", accountPrefix),
                    new XElement(Ns.Response + "AccountNumber", accountNumber),
                    new XElement(Ns.Response + "BankCode", bankCode));
            }
        }

        class NonstandardAccount : AccountType
        {
            readonly string accountNumber;

            NonstandardAccount(string accountNumber)
            {
                this.accountNumber = accountNumber;
            }

            public static NonstandardAccount Parse(NestandardniUcetType ucet)
            {
                return new NonstandardAccount(ucet.cislo);
            }

            public override XElement ToXElement()
            {
                return new XElement(Ns.Response + "AccountTypeNonstandard",
                    new XElement(Ns.Response + "AccountNumber", accountNumber));
            }
        }
    }