If you want to fetch the entire latest contacts. 

First of all, open your `App.xaml.cs`, Use ` IContactsService contactsService;`  to get all of contacts, Here is all of `App.xaml.cs` code .

```
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace ContactsDemo
{
    public partial class App : Application
    {
        IContactsService contactsService;
        public App(IContactsService contactsService)
        {
            InitializeComponent();
            this.contactsService = contactsService;
            MainPage = new MainPage(contactsService);
        }

        protected override void OnStart()
        {
            // Handle when your app starts
        }

        protected override void OnSleep()
        {
            // Handle when your app sleeps
        }

        protected async override void OnResume()
        {
           //get all of contacts
            List<Contact> contacts =await  contactsService.RetrieveContactsAsync() as List<Contact>;

          
            MessagingCenter.Send<App, List<Contact>>(App.Current as App, "OneMessage", contacts);
        }
    }
}

```

Then open your `MainPage.xaml.cs`, We can send contacts(did not add new records) to `AddContactPage`.  Here is all of code about `MainPage.xaml.cs`.

```
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace ContactsDemo
{
    // Learn more about making custom code visible in the Xamarin.Forms previewer
    // by visiting https://aka.ms/xamarinforms-previewer
    [DesignTimeVisible(false)]
    public partial class MainPage : ContentPage
    {
        ContactsViewModel cvm;
        //IContactsService service;
        public List<Contact> contactList;
        IContactsService contactsService;
        public MainPage(IContactsService contactService)
        {
            InitializeComponent();
          
            this.contactsService = contactService;
            contactList = new List<Contact>();
            //service = contactService;
            BindingContext = new ContactsViewModel(contactService);
        }

        //void OnToggledEvent(object sender, EventArgs args)
        //{
        //    //cvm = new ContactsViewModel(service);
        //    //if (!Utility.search)
        //    //{
        //        ViewCell cell = (sender as Xamarin.Forms.Switch).Parent.Parent as ViewCell;
        //        if (cell.BindingContext is Contact)
        //        {
        //            Contact contact = cell.BindingContext as Contact;
        //            if (contact != null)
        //            {
        //                if (contact != null && !contactList.Contains(contact))
        //                {
        //                    contactList.Add(contact);
        //                }
        //                else if (contact != null && contactList.Contains(contact))
        //                {
        //                    contactList.Remove(contact);
        //                }
        //            }
        //        }
        //        Debug.WriteLine("contactList:>>" + contactList.Count);
        //    //}
        //}

        private async void AddNewContact(object sender, EventArgs e)
        {
            List<Contact> contactList22= await contactsService.RetrieveContactsAsync() as List<Contact>;
            await Navigation.PushModalAsync(new AddContactPage(contactList22));
        }
    }
}

```

Then open `AddContactPage.xaml.cs`.  Compare two list(add new record and not add new record list), we can  fetch the entire latest contacts in `MessagingCenter.Subscribe` method.


```
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace ContactsDemo
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class AddContactPage : ContentPage
    {
        public AddContactPage(List<Contact> contactList)
        {
            InitializeComponent();
            ContactList = contactList;
            //MessagingCenter.Subscribe<App>(App.Current as App, "isContactAdded", (sender) =>
            //{

            //    //How I can fetch the entire latest contacts here
            //    //After fetching conatcts only I can check the new phone number is added to the device phonebook
            //});


            MessagingCenter.Subscribe<App, List<Contact>>(App.Current, "OneMessage", (snd, arg) =>
            {
                Device.BeginInvokeOnMainThread(() =>
                {
                    //Navigation.PushAsync(new NavigationPage(new DetailsInfo(arg)));
                    Debug.WriteLine("I am here");
                    var AllNewContacts = arg as List<Contact>;

                    // can fetch the entire latest contacts here
                    var exp2 = AllNewContacts.Where(a => !ContactList.Exists(t => a.Name.Contains(t.Name))).ToList();

                });
            });
        }

        

        public List<Contact> ContactList { get; }

        private async void AddtoPhonebook(object sender, EventArgs e)
        {
            var phnumber = number_entry.Text;
            if (phnumber.Length > 1)
            {
                DependencyService.Get<IContact>().SaveContacts(" ", phnumber, " ");
            }
            else
            {
                await DisplayAlert("Alert", "Please enter vaild Phone number.", "Ok");
            }
        }
    }
}
```