Question

I have a list like this:

List<Controls> list = new List<Controls>

How to handle adding new position to this list?

When I do:

myObject.myList.Add(new Control());

I would like to do something like this in my object:

myList.AddingEvent += HandleAddingEvent

And then in my HandleAddingEvent delegate handling adding position to this list. How should I handle adding new position event? How can I make this event available?

Answer 1

You could inherit from List and add your own handler, something like

using System;
using System.Collections.Generic;

namespace test
{
    class Program
    {

        class MyList<T> : List<T>
        {

            public event EventHandler OnAdd;

            public new void Add(T item) // "new" to avoid compiler-warnings, because we're hiding a method from base-class
            {
                if (null != OnAdd)
                {
                    OnAdd(this, null);
                }
                base.Add(item);
            }
        }

        static void Main(string[] args)
        {
            MyList<int> l = new MyList<int>();
            l.OnAdd += new EventHandler(l_OnAdd);
            l.Add(1);
        }

        static void l_OnAdd(object sender, EventArgs e)
        {
            Console.WriteLine("Element added...");
        }
    }
}

Warning

1. Be aware that you have to re-implement all methods which add objects to your list. AddRange() will not fire this event, in this implementation.

2. We did not overload the method. We hid the original one. If you Add() an object while this class is boxed in List<T>, the event will not be fired!

MyList<int> l = new MyList<int>();
l.OnAdd += new EventHandler(l_OnAdd);
l.Add(1); // Will work

List<int> baseList = l;
baseList.Add(2); // Will NOT work!!!