Cast generic to derived type

Hi, I'm hoping someone could help solve this problem I've run into. I'm trying to cast a GenericCollection<Car> to a derived class which is essentially the same thing. The compiler seems happy, but I get a runtime error saying that its not possible, but it doesn't make sense. Surely CarCollection would have all the methods/properties/structures that its parent has I've cut down the code to make it as basic as possible. To run it, just instantiate a new Foo.testInheritance();

The ultimate goal is I want to add a Clone() method to the GenericCollection and reuse it in the inherited class. How can I go about doing this

using System;
using System.Collections.Generic;
using System.Text;
using System.ComponentModel;

namespace Foo
{
public class GenericCollection<T> : BindingList<T>
{
}

public class CarCollection : GenericCollection<Car>
{
}

public class Car
{
}

class testInheritance
{
public testInheritance()
{
GenericCollection<Car> gcar = new GenericCollection<Car>();

// error "Unable to cast object of type Foo.GenericCollection`1' to type 'Foo.CarCollection'.
CarCollection cc = (CarCollection)gcar;
}
}
}

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Cast generic to derived type

Andrew Raymond

Well, you can't cast something to a derived class. That's like writing:

object o = new object();
string s = (String) o;

Even though in your case, the two types are a bit more alike :)

Anyway, I can imagine you want to write a Clone() method for all subclasses of GenericCollection. This is a bit more involved though than just casting it, but can work like this:

public class GenericCollection<T,TList> : BindingList<T>
where TList : GenericCollection<T,TList>,new() {

public TList Clone() {
TList result = new TList();
// <copy items>
return result;
}
}
public class CarCollection : GenericCollection<Car, CarCollection> {}

Ramakrishna Neela
Thanks for clearing it up and offering that suggestion of using the generic TList recursively.. it didn't occur to me you could do that but it solves the problem perfectly.

Cheers :)
office of technology
The reason the compiler doesn't complain is because this modification would be ok:

public testInheritance()
{
GenericCollection<Car> gcar = new CarCollection();

// error "Unable to cast object of type Foo.GenericCollection`1' to type 'Foo.CarCollection'.
CarCollection cc = (CarCollection)gcar;
}

You can cast an object reference to a derived type if and only if the object actually IS of that type (the reference can be of that type or any parent class of that type).
Cast generic to derived type

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