问CoVariance反差

EN

interface IGetElement<out T>
{
    int Counter { get; }
    T GetNext();
}

interface IAddElement<in T>
{
    void Add(T t);
}

class Collection<T> : IAddElement<T>, IGetElement<T> where T : Fruit
{
    List<T> tList = new List<T>();

    private int _counter = 0;

    public int Count { get { return tList.Count; } }

    public void Add(T t)
    {
        tList.Add(t);
    }

    int IGetElement<T>.Counter
    {
        get { return _counter; }
    }

    public T GetNext()
    {
        return tList[_counter++];
    }

    public void Rset()
    {
        _counter = 0;
    }
}

abstract class Fruit
{
    public abstract void Taste();
}

class Apple : Fruit
{
    public override void Taste()
    {
        Console.WriteLine("Like Apple");
    }
}

 static void Main(string[] args)
 {
        IGetElement<Fruit> covarience = new Collection<Apple>(); // CoVarience..

        IAddElement<Apple> contravarience = new Collection<Fruit>();//ContraVarience.. Compiling fine and working also fine... :)

        IGetElement<Fruit> fruits = new Collection<Apple>();

        IAddElement<Fruit> apples2 = fruits as IAddElement<Apple>;//Here its Compiler error : Cannot implicitly convert type 'Test.IAddElement<Test.Apple>' to 'Test.IAddElement<Test.Fruit>'. An explicit conversion exists (are you missing a cast?)  

        IAddElement<Apple> apples1 = fruits as IAddElement<Fruit>;//This is not posible  
        /* Why this is not possible..? In this case No Compiler Error But Run Time error ie., 
       apples1 is NULL.. I don't know why.. because.. in fruits cantains apple only but 
       It is unable it caste it as ContrVariantly..  ?------------1 */

        IAddElement<Apple> apples = fruits as IAddElement<Apple>;//This is posible 


        IGetElement<Fruit> f = apples as IGetElement<Apple>;//This is posible.. 
        /* Why this is posible.. ? here we are casting t as CoVariantly..
        If ------------1 is not posible how this would be posible... yes.. I am casting to    
        actual object which is not Null.. ? -----------2 */
  }