Model-View-ViewModel in WPF

using System;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Globalization;
using System.IO;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Formatters.Binary;
using System.Security.Permissions;

namespace TaskList.DataModel
{
    [Serializable]
    public sealed class TaskCollection : ObservableCollection<Task>, ISerializable
    {
        [NonSerialized]
        private bool isDirty;

        public TaskCollection()
        {
        }

        [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Performance", "CA1805:DoNotInitializeUnnecessarily")]
        private TaskCollection(SerializationInfo info, StreamingContext context)
        {
            int count = info.GetInt32("Count");
            for (int i = 0; i < count; ++i)
            {
                Task task = (Task)info.GetValue(string.Format(CultureInfo.InvariantCulture, "Item{0}", i), typeof(Task));
                Add(task);
            }
            isDirty = false;
        }

        public bool IsDirty
        {
            get { return isDirty; }
            set
            {
                if (isDirty != value)
                {
                    isDirty = value;
                    OnPropertyChanged(new PropertyChangedEventArgs("IsDirty"));
                }
            }
        }

        public static TaskCollection Load(string fileName)
        {
            using (FileStream stream = new FileStream(fileName, FileMode.Open))
            {
                BinaryFormatter binaryFormatter = new BinaryFormatter();
                return binaryFormatter.Deserialize(stream) as TaskCollection;
            }
        }

        public void Save(string fileName)
        {
            using (FileStream stream = new FileStream(fileName, FileMode.Create))
            {
                BinaryFormatter binaryFormatter = new BinaryFormatter();
                binaryFormatter.Serialize(stream, this);
            }
        }

        protected override void InsertItem(int index, Task item)
        {
            base.InsertItem(index, item);
            AddChangeListener(item);
            IsDirty = true;
        }

        protected override void RemoveItem(int index)
        {
            Task item = this[index];
            base.RemoveItem(index);
            RemoveChangeListener(item);
            IsDirty = true;
        }

        protected override void ClearItems()
        {
            if (Count > 0)
            {
                foreach (Task item in this)
                    RemoveChangeListener(item);
                base.ClearItems();
                IsDirty = true;
            }
        }

        protected override void SetItem(int index, Task item)
        {
            if (this[index] != item)
            {
                RemoveChangeListener(this[index]);
                base.SetItem(index, item);
                AddChangeListener(item);
                IsDirty = true;
            }
        }

        protected override void MoveItem(int oldIndex, int newIndex)
        {
            base.MoveItem(oldIndex, newIndex);
            IsDirty = true;
        }

        private void AddChangeListener(Task item)
        {
            if (item != null)
                item.PropertyChanged += item_PropertyChanged;
        }

        private void RemoveChangeListener(Task item)
        {
            if (item != null)
                item.PropertyChanged -= item_PropertyChanged;
        }

        private void item_PropertyChanged(object sender, PropertyChangedEventArgs e)
        {
            IsDirty = true;
        }

        #region ISerializable Members

        [SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.SerializationFormatter)]
        public void GetObjectData(SerializationInfo info, StreamingContext context)
        {
            info.AddValue("Count", Count);
            for (int i = 0; i < Count; ++i)
            {
                info.AddValue(string.Format(CultureInfo.InvariantCulture, "Item{0}", i), this[i]);
            }
        }

        #endregion
    }
}