集合中的C＃覆盖方法<T>

Question

I would like to notify to the user each time that the program add a new Item in a List.

I did this:

public class ListDemo<T> : Collection<T>
{
    protected override void InsertItem(int index, T item)
    {
        Console.WriteLine("YOU HAVE INSERTED AN ITEM");
        base.InsertItem(index, item);
    }
}

I could inherit ListDemo from the interface IList , but I don't want to implement all the methods of this interface... So I inherited from Collection

Now I have two questions about my code:

1º Do I have to notify to the user with an event? Or my code is good?

2º How Can I call to the override method? Like an Interface?

         // IT WORKS!
         List<int> _list = new ListDemo<int>();
        _list.Insert(0,2);
        Console.WriteLine(_list[0]);

This code works, and it is very strange for me, because when you call to an interface you must use this way:

        Collection<int> _list = new ListDemo<int>();

3º Why if the override method is InsertItem , then when I call to the method from my instance this is simply Insert .

Answer 1

If you implementation does what you want, there is no problem. That being said, there is already a predefined type ObservableCollection<T> , which is documented here , which does exactly what you want, namely providing a mechanism for notifications if the content of the collections changes.

Answer 2

The InsertItem is a virtual method of Collection<T> :

protected virtual void InsertItem(
    int index,
    T item
)

Please have a look here . So when a type inherits a Collection<T> then it can override this method and when InsertItem is called the overriden one it would be called.

On the other hand List<T> doesn't not inherits Collection<T> so there isn't any method called InsertItem .

That being said, defining you object as below:

var listDemo = new ListDemo<int>();
listDemo.InsertItem(0,2);

you would achieve that you want, because now the type of listDemo inherits from Collection<int> and you have overriden this method.