Converting a List of Base type to a List of Inherited Type

Question

I would be certain that this question addresses something that would have been brought up in a previous question, but I was unable to find it.

There is a method in a

Answer 1

You cannot do that. To understand why it is not allowed, imagine what would happen if Add was called on a List<Derived> after it had been cast to a List<Base>.

Also, the answers implying that C# 4.0 will be different are wrong. List will never be modified to allow you to do this. Only IEnumerable will - because it does not allow items to be added to the collection.

Update: The reason it works in the solution you've gone for is because you're no longer passing the same list. You're creating a whole new list which is a copy of the original. This is why I asked about modifying the list; if MethodC makes changes to the number of items on the list, those changes would be made to a copy, not the original list.

I think the ideal solution for you is as follows:

public abstract class A
{
    public void MethodC<TItem>(List<TItem> list) where TItem : A
    {
        foreach (var item in list)
            item.CanBeCalled();
    }

    public abstract void CanBeCalled();
}

public class B : A
{
    public override void CanBeCalled()
    {
        Console.WriteLine("Calling into B");
    }
}

class Program
{
    static void Main(string[] args)
    {
        List<B> listOfB = new List<B>();

        A a = new B();

        a.MethodC(listOfB);
    }
}

Notice how, with this solution, you can pass a List<B> directly to MethodC without needing to do that weird conversion on it first. So no unnecessary copying.

The reason this works is because we've told MethodC to accept a list of anything that is derived from A, instead of insisting that it must be a list of A.

Answer 2

Here is an answer that will exclude any objects in your list of the wrong type. This is a much safer way in my opinion:

List<A> ListOfA  = ListOfB.OfType<A>().ToList();

The OfType method will exclude items of the wrong derived class where as the Cast will throw an error.

Answer 3

If you have linq available you can do

var ListOfA = ListOfB.Cast<A>().ToList();

Answer 4

You are addressing the lack of covariance in the current C# version. Here is one way of doing it:

listOfB.Cast<A>();