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public abstract class AbstractCollection<T> : List<T>
{
    public delegate void OnItemAddedEventHandler(T item);

    public event OnItemAddedEventHandler OnItemAdded;

    public new void Add(T item)
    {
        // Custom implementation
        if (OnItemAdded != null)
        {
            OnItemAdded(item);
        }

        base.Add(item);
    }

    public new void Remove(T item)
    {
        // Custom implementation

        base.Remove(item);
    }

    public new void RemoveAt(int index)
    {
        // Custom implementation

        base.RemoveAt(index);
    }
}