如何创建具有继承的泛型类？

如何使以下代码有效？我不认为我完全理解C＃泛型。也许，有人可以指出我正确的方向。

How can I make the following code work? I don't think I quite understand C# generics. Perhaps, someone can point me in the right direction.

    public abstract class A
    {
    }

    public class B : A
    {
    }

    public class C : A
    {
    }

    public static List<C> GetCList()
    {
        return new List<C>();
    }

    static void Main(string[] args)
    {
        List<A> listA = new List<A>();

        listA.Add(new B());
        listA.Add(new C());

        // Compiler cannot implicitly convert
        List<A> listB = new List<B>();

        // Compiler cannot implicitly convert
        List<A> listC = GetCList();

        // However, copying each element is fine
        // It has something to do with generics (I think)
        List<B> listD = new List<B>();
        foreach (B b in listD)
        {
            listB.Add(b);
        }
    }

这可能是一个简单的答案。

It's probably a simple answer.

更新：
首先，这在C＃3.0中是不可能的，但在C＃4.0中是可能的。

Update: First, this is not possible in C# 3.0, but will be possible in C# 4.0.

要使它在C＃3.0中运行，这只是一个解决方法，直到4.0，请使用以下内容：

To get it running in C# 3.0, which is just a workaround until 4.0, use the following:

        // Compiler is happy
        List<A> listB = new List<B>().OfType<A>().ToList();

        // Compiler is happy
        List<A> listC = GetCList().OfType<A>().ToList();