Practices about Data Access Object Pattern

Data Access Object.cs

public class Account
{ 
    // set fields and properties
}

public class Customer
{
    // set fields and properties
}

public interface IAccount
{
    int CreateAccount(Account account);
    bool DeleteAccount(int accountId);
    bool UpdateAccount(Account account);
    List<Account> GetAccount();
}

public interface ICustomer
{
    int CreateCustomer(Customer customer);
    bool DeleteCustomer(int customerId);
    bool UpdateCustomer(Customer customer);
    List<Customer> GetCustomers();
    bool DeleteAccounts(int customerID);
}

// Data Account Object for sql connect
public class SqlAccountDAO
    : IAccount
{
    public int CreateAccount(Account account)
    {
        throw new NotImplementedException();
    }

    public bool DeleteAccount(int accountId)
    {
        throw new NotImplementedException();
    }

    public bool UpdateAccount(Account account)
    {
        throw new NotImplementedException();
    }

    public List<Account> GetAccount()
    {
        throw new NotImplementedException();
    }
}

public class SqlCustomerDAO
    : ICustomer
{
    public int CreateCustomer(Customer customer)
    {
        throw new NotImplementedException();
    }

    public bool DeleteCustomer(int customerId)
    {
        throw new NotImplementedException();
    }

    public bool UpdateCustomer(Customer customer)
    {
        throw new NotImplementedException();
    }

    public List<Customer> GetCustomers()
    {
        throw new NotImplementedException();
    }

    public bool DeleteAccounts(int customerID)
    {
        throw new NotImplementedException();
    }
}

// Data Account Object for oracle connect
public class OracleAccountDAO
    : IAccount
{
    public int CreateAccount(Account account)
    {
        throw new NotImplementedException();
    }

    public bool DeleteAccount(int accountId)
    {
        throw new NotImplementedException();
    }

    public bool UpdateAccount(Account account)
    {
        throw new NotImplementedException();
    }

    public List<Account> GetAccount()
    {
        throw new NotImplementedException();
    }
}

public class OracleCustomerDAO
    : ICustomer
{
    public int CreateCustomer(Customer customer)
    {
        throw new NotImplementedException();
    }

    public bool DeleteCustomer(int customerId)
    {
        throw new NotImplementedException();
    }

    public bool UpdateCustomer(Customer customer)
    {
        throw new NotImplementedException();
    }

    public List<Customer> GetCustomers()
    {
        throw new NotImplementedException();
    }

    public bool DeleteAccounts(int customerID)
    {
        throw new NotImplementedException();
    }
}

// Data Access Object Factory
public abstract class DAOFactory
{
    public abstract ICustomer GetCustomerDao();
    public abstract IAccount GetAccountDao();

    public static DAOFactory GetDaoFactory()
    {
        string strDaoFactoryType = ConfigurationManager.AppSettings["DAOFactory"];
        if (String.IsNullOrEmpty(strDaoFactoryType))
            throw new NullReferenceException("");
        else
        {
            Type DaoFactoryType = Type.GetType(strDaoFactoryType);
            if (DaoFactoryType == null)
                throw new NullReferenceException("");

            Type DaoFactoryBaseType =
                    Type.GetType("Sample.DataAccessLayer.DAOFactory");
            if (!DaoFactoryBaseType.IsAssignableFrom(DaoFactoryType))
                throw new ArgumentException("");

            DAOFactory daoFactory = (DAOFactory)Activator.CreateInstance(DaoFactoryType);
            return daoFactory;
        }
    }
}

public class SqlDAOFactory
    : DAOFactory
{
    public static string ConnectionString
    {
        get
        {
            if (ConfigurationManager.ConnectionStrings["MsSqlDao"] == null)
                throw new NullReferenceException("");
            string connectionString =
                    ConfigurationManager.ConnectionStrings["MsSqlDao"].ConnectionString;
            if (String.IsNullOrEmpty(connectionString))
                throw new NullReferenceException("");
            else
                return connectionString;
        }
    }

    public override ICustomer GetCustomerDao()
    {
        return new SqlCustomerDAO();
    }

    public override IAccount GetAccountDao()
    {
        return new SqlAccountDAO();
    }
}

public class OracleDAOFactory
    : DAOFactory
{
    public static string ConnectionString
    {
        get
        {
            if (ConfigurationManager.ConnectionStrings["OracleDao"] == null)
                throw new NullReferenceException("");
            string connectionString =
                    ConfigurationManager.ConnectionStrings["OracleDao"].ConnectionString;
            if (String.IsNullOrEmpty(connectionString))
                throw new NullReferenceException("");
            else
                return connectionString;
        }
    }

    public override ICustomer GetCustomerDao()
    {
        throw new NotImplementedException();
    }

    public override IAccount GetAccountDao()
    {
        throw new NotImplementedException();
    }
}