How to create a derived object based on base type only?

Question

For example I have this classes:

public abstract class Device 
{
    public Device()
    {
    }

    public Device(XmlDevice xmlDevice)
    {
        // creating device by xmlDevice
    }
} 

public class Modem : Device
{
    public Modem()
    {
    }

    public Modem(XmlModem modem)
    {
        // creating modem by xmlModem
    }
}

public class Plc : Device
{
    public Plc()
    {
    }

    public Plc(XmlPlc plc)
    {
        // creating plc by xmlPlc
    }
}


public abstract class XmlDevice 
{
    public XmlDevice()
    {
    }
} 

public class XmlModem : XmlDevice
{
    public XmlModem()
    {
    }
}

public class XmlPlc : XmlDevice
{
    public XmlPlc()
    {
    }
}

I have a list with XmlDevice objects - how can I create Device objects based on its type? I can do this like this:

foreach( xmlDevice in xmlDevicesList )
{
    if( xmlDevice is XmlModem )
        devicesList.Add( new Modem((XmlModem)xmlDevice) );
    else if( xmlDevice is XmlPlc )
        devicesList.Add( new Plc((XmlPlc)xmlDevice) );
}

But I want to avoid "is" statement and casting objects. Is it more clean way to do this?

Please have a look at the response to similar question I asked a few years ago: http://stackoverflow.com/questions/15998071/getting-past-open-closed-principle — Kamil T, Jun 13 '16 at 08:28

Martin Törnwall · Accepted Answer · 2016-06-13T11:03:57.093

4

Your aversion to your current implementation is well-founded. I can promise you it's going to break the minute you add a new device type. What's worse, the compiler has no way of knowing that you intended the instance type check to be exhaustive, and therefore won't help you by signalling an error.

If you can change the Xml* classes, you could simply introduce an abstract method CreateDevice in XmlDevice, and override it in the derived classes:

public abstract class XmlDevice
{
    // ...
    public abstract Device CreateDevice();
    // ...
}

public class XmlModem : XmlDevice
{
    // ...
    public override Device CreateDevice()
    {
        return new Modem(this);
    }
    // ...
}

Now you can simply do this:

devicesList = xmlDevicesList.ConvertAll(x => x.CreateDevice());

Perhaps the greatest benefit of this approach is that the compiler won't let you get away with creating a new device type without implementing the CreateDevice method.

edited Jun 13 '16 at 11:03

answered Jun 13 '16 at 10:41

Martin Törnwall

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Thanks, this is so simple... If I want to pass some different parameters (different for Modem and Plc) to constructors then factory would be necessary? – Bero Jun 13 '16 at 11:32
What parameters, for example? – Martin Törnwall Jun 13 '16 at 11:44
Some other objects. For example: Antena for Modem and OutputModule for Plc – Bero Jun 16 '16 at 10:31

How to create a derived object based on base type only?

1 Answers1