C# Dictionary change notification (INotifyCollectionChanged)

Question

I have a loop running retrieving live share prices. What I want to do is check if any of the prices retrieved are different to the price I already have stored in a dictionary and notify me with details of all those that have changed. Was looking along the lines of Dictionary utilising INotifyCollectionChanged.

e.g.

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;

namespace BasicExamples
{
    class CollectionNotify:INotifyCollectionChanged
    {
        public Dictionary<string, string> NotifyDictionary{ get; set; }     

        public CollectionNotify()
        {
            NotifyDictionary = new Dictionary<string, string>();
        }

        public event NotifyCollectionChangedEventHandler CollectionChanged;

        protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
        {
            if (CollectionChanged != null)
            {
                CollectionChanged(this, e);
            }
        }

        public void Add(object k, object v)
        {
            NotifyDictionary.Add(k.ToString(),v.ToString());
            OnCollectionChanged(new     NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add,0));
        }

        public void Update(object k, object v)
        {
            bool isUpdated = false;
            IList<string> changedItems = new List<string>();
            int index;
            if (NotifyDictionary.ContainsKey(k.ToString()))
            {
                NotifyDictionary[k.ToString()] = v.ToString();
                changedItems.Add(k+":"+v);
                isUpdated = true;
            }
            else
            {
                Add(k, v);
            }
            if (isUpdated)
            {
                OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Replace,changedItems,changedItems));
            }                
        }
    }
}

with the main program calling Add/Update as a test. However when looping through the NotifyCollectionChangedEventArgs I have to nest the loop and cast to IEnumerable - this seems like a really long winded way round.

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace BasicExamples
{
    class Program
    {
        //delegate 
        static void Main(string[] args)
        {
            //notify collection
            CollectionNotify notify = new CollectionNotify();
            notify.CollectionChanged += CollectionHasChanged;

            notify.Add("Test1", "Test2");
            notify.Add("Test2","Test2");
            notify.Add("Test3", "Test3");
            notify.Update("Test2", "Test1");
            notify.Update("Test2", "Test3");
            notify.Update("Test3", "Test1");
            notify.Update("Test1", "Test3");

            #region Lamba

            //lamba
            List<int> myList = new List<int>() {1,1,2,3,4,5,6};

            List<int> newList = myList.FindAll(s =>
            {
                if (s == 1)
                {
                    return true;
                }
                return false;
            });

            foreach (int b in newList)
            {
                Console.WriteLine(b.ToString());
            }

            #endregion
        }

        public static void CollectionHasChanged(object sender, EventArgs e)
        {
            NotifyCollectionChangedEventArgs args = (NotifyCollectionChangedEventArgs) e;
            if (args.Action == NotifyCollectionChangedAction.Replace)
            {
                foreach (var item in args.NewItems)
                {
                    foreach (var nextItem in (IEnumerable)item)
                    {
                        Console.WriteLine(nextItem);
                    }
                }
            }
        }
    }
}

I suppose my question is two fold:- A: Is this the best way to use a notifiable dictionary? B: Should i use the nested loop as above to retrieve the updated values?

Thanks in advance