How to handle add to list event?

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!!!

I believe What you're looking for is already part of the API in the ObservableCollection(T) class. Example:

ObservableCollection<int> myList = new ObservableCollection<int>();

myList.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(
    delegate(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)                    
    {
        if (e.Action == System.Collections.Specialized.NotifyCollectionChangedAction.Add)
        {
            MessageBox.Show("Added value");
        }
    }
);

myList.Add(1);