У меня есть небольшие сомнения в отношении CoVariance и ContraVariance .. См. Следующий код ..
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");
}
}
Это пример кода .. И теперь Клиент
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 */
}
Пожалуйста, ответьте на мой вопрос в закомментированном исходном коде ... :) -------- 1, --------- 2.
Спасибо и привет, Динеш