How to get a new copy from a polymorfic list?

Stig Gunnarsson

I have a base class Car. From this I derive some 25 classes of CarTypes (Ford, BMW and so on), and from these I derive some 10 classes of carModels.

Now I have a List cars, that can contain 250 different carModel instances. I want to retreive a new instance of the carModel at position index in list, but NOT an EMPTY instance but a COPY of the carModel at cars[index].

All of the 250 CarModel classes has a copy constructor. I can use reflection to get the type

Type t = (Typeof) cars[index]; Then I can create an EMPTY instance with

activator.CreateInstance(t); but HOW do I invoke the copy constructor of the element at position index for copy OR can I use get properties to work on a list of Car?

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Tapan Patel9y ago
Accepted answer
I found your issue interesting. Here is the proof of what you are looking for.
  public interface IDemo { }

/// class implementing interface
    public class DemoX:IDemo
    {
        string name;
        public DemoX(string name)
        {
            this.name = name;
            Console.WriteLine($"This is {this.name} constructor");

        }
        public DemoX(DemoX obj)
        {
            this.name = obj.name;
            Console.WriteLine($"This is copy of {name} constructor");
        }
    }
/// Another class implementing interface
    public class DemoY : IDemo
    {
        string name;
        public DemoY(string name)
        {
            this.name = name;
            Console.WriteLine($"This is {this.name} constructor");

        }
        public DemoY(DemoY obj)
        {
            this.name = obj.name;
            Console.WriteLine($"This is copy of {name} constructor");
        }
    }
Inside the calling method, I have list of different objects of classes implementing this interface and then I am trying to call the copy constructor of the 0th Index object.
Key is to get find the parameterized constructor(which in your case whould be copy constructor) and invoke it using Invoke() method.

check this code out the way you like.
   static void Main(string[] args)
        {


            List lst = new List();
            lst.Add(new DemoX("A"));
            lst.Add(new DemoY("B"));
            lst.Add(new DemoX("C"));
            lst.Add(new DemoY("D"));
            lst.Add(new DemoX("E"));

            IDemo d = lst[0];
            Type t = lst[0].GetType();

            ConstructorInfo ctor = t.GetConstructor(new[] { t });

            object newObj = ctor.Invoke(new object[] { lst[0] });
}
Stig Gunnarsson9y ago
Thanks for answer. I found out that there is also another way, just like the answer.
IF the objects in list HAVE a copy constructor, I can get a new object (Car) like:
(car)Activator.CreateInstance(cars[index].GetType(), cars[index]);
creating a copy of the actual car at position index in cars list.
Tapan Patel9y ago
Instead of doing copy constructor, why don't you go with extension method. That way, you can directly call method on current object. Also, don't forget to add that extension method through Car interface.