How do I upcast a collection of base class in C#?

Question

In C#, I have a class A that I cannot modify. From this class I've created class B (i.e. inherited from A) which adds some properties. I also have a static function that returns List<A> when GetListOfA() is called.

How can I cast the return from GetListOfA() to List<B>?

e.g. List<B> bList = foo.GetListOfA() as List<B>

Answer 1

If you're using C# 3.0 (Visual Studio 2008), you can:

using System.Linq;

List<B> bList = foo.GetListOfA().Cast<B>().ToList();

Answer 2

Using System.Linq, you can say

List<B> bList = foo.GetListOfA().Cast<B>().ToList()

Or

var bs = foo.GetListOfA().Cast<B>()

Or

static class AExts
{
    public List<B> AsB( this List<A> list )
    {
        return list.Cast<B>().ToList();
    }
}

List<B> bList = foo.GetListOfA().AsB();

Or

static class FooExts
{
    public List<B> AsB( this Foo foo )
    {
        return list.GetListOfA().AsB();
    }
}
List<B> bList = foo.AsB();

Or if you dont have/ cant /wont use Linq, algorithms in PowerCollections has the same thing

And I didnt look at the other answer even though it probably says the same!

Answer 3

You can't. You must actually create a new List<B> and add all the items from the original list to the new list.

The easiest way to do this, apart from writing your own method to do so, is to use Linq (see other answers).