Avoiding parallel inheritance hierarchies

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I am thinking of using the Visitor pattern.

public class Car : Vehicle
{
   public void Accept( IVehicleFormatter v )
   {
       v.Visit (this);
   }
}

public class Truck : Vehicle
{
   public void Accept( IVehicleFormatter v )
   {
       v.Visit (this);
   }
}

public interface IVehicleFormatter
{
   public void Visit( Car c );
   public void Visit( Truck t );
}

public class VehicleXmlFormatter : IVehicleFormatter
{
}

public class VehicleSoapFormatter : IVehicleFormatter
{
}

With this, you avoid an extra inheritance tree, and keep the formatting logic separated from your Vehicle-classes.
Offcourse, when you create a new vehicle, you’ll have to add another method to the Formatter interface (and implement this new method in all the implementations of the formatter interface).
But, I think that this is better then creating a new Vehicle class, and for every IVehicleFormatter you have, create a new class that can handle this new kind of vehicle.

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