C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    abstract class A
    {    }

    class B : A
    {    }

    class C : A
    {    }


    class ClassX
    {
        static void DoStuff()
        {
            List<B> listB = new List<B>();
            List<C> listC = new List<C>();

            List<A>[] arrayOfLists = new List<A>[2];

            arrayOfLists[0] = listB;//Error	1	Cannot implicitly convert type 'System.Collections.Generic.List<ConsoleApplication1.B>'
            arrayOfLists[1] = listC;//Error	2	Cannot implicitly convert type 'System.Collections.Generic.List<ConsoleApplication1.C>'

            arrayOfLists[0] = listB.Cast<A>().ToList();//Subsequent changes in list B are not seen an arrayoflists because a new object is created

            List<List<A>> listOfLists = new List<List<A>>();
            listOfLists.Add(listB);//Error	4	Argument 1: cannot convert from 'System.Collections.Generic.List<ConsoleApplication1.B>' to 'System.Collections.Generic.List<ConsoleApplication1.A>'
        }
    }
}