An Address Book Application Made in MVVM For Metro Style App in Windows 8 Part 2

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;
using Mvvm3_Basic.Model;

namespace Mvvm3_Basic.ViewModel
{
    public class ContactViewModel : INotifyPropertyChanged
    {
        public ContactViewModel()
        {
            AddCommand = new MyCommand<object>(OnAdd);
            SaveCommand = new MyCommand<object>(OnSave);
            UpdateCommand = new MyCommand<object>(OnUpdate);
            DeleteCommand = new MyCommand<object>(OnDelete);
            GetCommand = new MyCommand<object>(GetContact);
        }

        public ObservableCollection<Contact> Contacts { get; set; }
        
        public MyCommand<object> AddCommand { get; set; }
        public MyCommand<object> SaveCommand { get; set; }
        public MyCommand<object> UpdateCommand { get; set; }
        public MyCommand<object> DeleteCommand { get; set; }
        public MyCommand<object> GetCommand { get; set; }

        void OnAdd(object obj)
        {
            Contact c = new Contact { Name = string.Empty, Email = string.Empty };
            SelectedContact = c;
            PropertyChanged(this, new PropertyChangedEventArgs("SelectedContact"));

            Debug.WriteLine("Add Called!");
        }

        void OnSave(object obj)
        {
            db.Save(SelectedContact);
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("SelectedContact"));

            Debug.WriteLine("Save Called!");
        }

        void OnUpdate(object obj)
        {
            db.Update(SelectedContact);
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Contacts"));

            Debug.WriteLine("Update Called!");
        }
        
        void OnDelete(object obj)
        {
            db.Delete(SelectedContact);
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("SelectedContact"));

            Debug.WriteLine("Delete Called!");
        }
        
        void GetContact(object obj)
        {
            if (Contacts == null) Contacts = new ObservableCollection<Contact>();
            Contacts.Clear();
            db.GetContactList();
            Contacts = db.contactlist;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Contacts"));

            Debug.WriteLine("Refresh called!");
        }

        DB db = new DB();    //at class level

        private Contact _ctc = new Contact();
        public Contact SelectedContact
        {
            get { return _ctc; }
            set
            {
                _ctc = value;
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("SelectedContact"));

            }
        }


        public event PropertyChangedEventHandler PropertyChanged;
    }
    public class MyCommand<T> : ICommand
    {
        readonly Action<T> callback;

        public MyCommand(Action<T> callback)
        {
            this.callback = callback;
        }

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public event EventHandler CanExecuteChanged;

        public void Execute(object parameter)
        {
            if (callback != null) { callback((T)parameter); }
        }
    }
}