public interface IMailService
{
    void Send(string message);
 
    string? Receive();
 
    IMailService CreateService(object userToken);
}

// IMailService를 구현하는 abstract class.
// interface를 구현하는 경우에도 추상화 할 수 있다.
 
public abstract class MailClient : IMailService
{
    protected IMailService? Service;
 
    public abstract void Send(string message);
 
    public abstract string? Receive();
 
    public abstract IMailService CreateService(object userToken);
}
 
 
// abstract를 구현하는 파생 class
 
public class ClientTypeA : MailClient
{
    public ClientTypeA(object userToken) => Service = CreateService(userToken);
 
    public override void Send(string message) => Service?.Send(message);
 
    public override string? Receive() => Service?.Receive();
 
    public override IMailService CreateService(object userToken)
    {
        var transport = new MailTransport(userToken);
        if (transport.Connect(userToken))
        {
            Service = transport;
            return transport;
        }
        else throw new HttpRequestException($"Invalid connection - {userToken}");
    }
}