Convert List of Base Class to a List of Derived Class - possible?

Question

I coworker of mine told me that it was possible to convert a List<BaseClass> to a List<DerivedClass> (they didn't show me a working example). I didn't think it was possible to convert a parent class into a child class (I do know however that it is possible to do it the other way around). I've seen several related questions with people saying it can't be done and people saying it can - none of the proposed solutions have worked for me at all. I always get a:

System.InvalidCastException.
Unable to cast object of type 'BaseClass' to 'ChildClass'

Is this absolutely possible? If so, what am I doing wrong? Here is my code (simplified for the purposes of this question):

Parent class:

public class ParentClass
{
    public string Name = "";
}

Child class:

public class ChildClass: ParentClass
{
    public string Date = "";
}

Converting:

List<ParentClass> parentList = ServiceApi.GetParentClassList().ToList();
List<ChildClass> childList = parentList.Cast<ChildClass>().ToList();

My actual code is significantly more complex but the same principles should apply.

Answer 1

In a sense, you're both right.

Let's say you have types Animal, Cat (inherits Animal) and Dog (inherits Animal). A List<Animal> can contain both Cats and Dogs, but a List<Dog> cannot contain Cats. If you have a List<Animal> that contains both Cat and Dog objects, nothing you do will ever let you produce a List<Dog> with everything from the original list.

So in that sense you are right that you cannot produce List<DerivedClass> from List<ParentClass>.

But there are times when you, as the programmer, have information about how an object is used that is not available to the compiler. If (and only if) you can know that a particular List<Animal> instance only contains Dog objects, you can always write something like this:

List<Dog> dogList = myAnimalListWithOnlyDogs.Cast<Dog>().ToList();

Again: this only works when you can guarantee every object in your list instance is convertible to a Dog object. If a Cat object somehow works it's way into your list, you'll end up with an exception at run time. Nevertheless, code like this is not uncommon.

In this sense, the coworker is right. When it comes to real working code, people do it anyway with great effect, and get away with it just fine, though a purist might argue this is a code smell indicating a need to use composition instead of inheritance.

Additionally, there may be times when a List<Animal> instance contains both Cat and Dog objects, and you only care about the Dog objects. In that case, you can do this:

List<Dog> dogList = myAnimalList.OfType<Dog>().ToList();

This pattern in especially common when working with WinForms controls.

In both examples above you are working with a new sequence. Adding or removing objects to the new sequence will not change the original. In this sense, you have not converted the original as much a created a replacement. However, the sequence contains the same objects, so editing a property on a Dog object in the new sequence will change that object in the original, because it's the same object instance.

Answer 2

Cast<T> method applies cast operation to all elements of the input sequence. It works only if you can do the following to each element of the sequence without causing an exception:

ParentClass p = ...
ChildClass c = (ChildClass)p;

This will not work unless p is assigned an instance of ChildClass or one of its subclasses. It appears that in your case the data returned from the server API contains objects of ParentClass or one of its subclasses other than ChildClass.

You can fix this problem by constructing ChildClass instances, assuming that you have enough information from the server:

List<ChildClass> childList = parentList
    .Select(parent => new ChildClass(parent.Name, ... /* the remaining fields */))
    .ToList();