NP110 Why is it significant

Hi Guys

NP110 Why is it significant

Why is it significant to check if (Changed != null)?. Because without this check   

Program is executing well. Please explain the reason.

Thank you

// events1.cs

using System;

namespace MyCollections

{

    using System.Collections;

    // A delegate type for hooking up change notifications.

    public delegate void ChangedEventHandler(object sender, EventArgs e);

    //carry the event data

    // A class that works just like ArrayList, but sends event

    // notifications whenever the list changes.

    public class ListWithChangedEvent : ArrayList

    {

        // An event that clients can use to be notified whenever the

        // elements of the list change.

        public event ChangedEventHandler Changed;

        // Invoke the Changed event; called whenever list changes

        protected virtual void OnChanged(EventArgs e)

        {

            if (Changed != null)

                Changed(this, e);

        }

        // Override some of the methods that can change the list;

        // invoke event after each

        public override int Add(object value)

        {

            int i = base.Add(value);

            OnChanged(EventArgs.Empty);

            return i;

        }

        public override void Clear()

        {

            base.Clear();

            OnChanged(EventArgs.Empty);

        }

        public override object this[int index]

        {

            set

            {

                base[index] = value;

                OnChanged(EventArgs.Empty);

            }

        }

    }

}

namespace TestEvents

{

    using MyCollections;

    class EventListener

    {

        private ListWithChangedEvent List;

        public EventListener(ListWithChangedEvent list)

        {

            List = list;

            // Add "ListChanged" to the Changed event on "List".

            List.Changed += new ChangedEventHandler(ListChanged);

        }

        // This will be called whenever the list changes.

        private void ListChanged(object sender, EventArgs e)

        {

            Console.WriteLine("This is called when the event fires.");

        }

        public void Detach()

        {

            // Detach the event and delete the list

            List.Changed -= new ChangedEventHandler(ListChanged);

            List = null;

        }

    }

    class Test

    {

        // Test the ListWithChangedEvent class.

        public static void Main()

        {

            // Create a new list.

            ListWithChangedEvent list = new ListWithChangedEvent();

            // Create a class that listens to the list's change event.

            EventListener listener = new EventListener(list);

            // Add and remove items from the list.

            list.Add("item 1");

            list.Clear();

            listener.Detach();

        }

    }

}

/*

This is called when the event fires.

This is called when the event fires.

*/