Transform class to a derived class

Question

Is there a way to transform a base class into its derived class?

Here is a simple example of two classes:

namespace BLL
{
    public class Contact 
    {
       public int ContactID { get; set; }
       public string Name { get; set; }

       public Contact(){}
    }
}

namespace BLL
{
    public class SpecialContact : Contact
    {
       public SpecialContact(){}
    }
}

Ideally, I could do something like this:

Contact contact = new Contact();
SpecialContact specialContact = new SpecialContact();

contact.ContactID = 123;
contact.Name = "Jeff";

specialContact = contact;

This code of course throws an error. Apart from writing another constructor for SpecialContact or method that sets each property, is there any other solution?

Answer 1

It is illegal to assign Base class reference to the Derived class reference variable.

Variable of type X can only be a reference to an object of type X or derived.

You probably need to have another instance of SpecialContact which contains the same data as existing object. There is no way to avoid manual copying.

I use the following extension method when I need to copy the matching properties from one object to another incompatible one(1):

public static void AssignFrom(this object destination, object source) {
  Type dest_type = destination.GetType();
  Type source_type = source.GetType();

  var matching_props = from d in dest_type.GetProperties()
                        join s in source_type.GetProperties()
                        on d.Name equals s.Name
                        where d.IsWritable() && s.IsReadable()
                        select new {
                          source = s,
                          destination = d
                        };

  foreach (var prop in matching_props) {
    prop.destination.SetValue(destination, prop.source.GetValue(source, null), null);
  }
}

Then you can do:

specialContact.assignFrom(contact);

Please consider this a workaround. The proper solution would be to design properly your class hierarchy where you do not come to this problem.

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1 Note: it matches the properties by name and assumes they are of the same type.