ObservableCollection<T>

Following on from the previous post where I noted that ObservableCollection<T>.CollectionChanged didn't do what I wanted, I've created my own version of ObservableCollection that exposes a new event ItemPropertyChanged. I then hook up to the PropertyChanged event of the business object as it is added to the collection. The new class adds a new property HasChanges which reflects the change status of any property in any business object in the collection.

Caveats:
The business object must implement INotifyPropertyChanged.
There is no "undo" mechanism to set the HasChanges flag back to false

public class MyObservableCollection : ObservableCollection
{
   public new void Add(T item)
   {
      // If the class represented by T implements the INotifyPropertyChanged
      // interface hook up the PropertyChanged event handler
      if ((item as INotifyPropertyChanged) != null)
      {
         (item as INotifyPropertyChanged).PropertyChanged += new PropertyChangedEventHandler(OnItemPropertyChanged);
      }
      base.Add(item);
   }

   public bool HasChanges { get; set; }
   public event PropertyChangedEventHandler ItemPropertyChanged;
   public MyObservableCollection()
   {
      HasChanges = false;
   }
   protected void OnItemPropertyChanged(object sender, PropertyChangedEventArgs e)
   {
      HasChanges = true;
      OnPropertyChanged("HasChanges");
   }
   protected void OnPropertyChanged(string propertyName)
   {
      if (ItemPropertyChanged != null)
      {
         ItemPropertyChanged(this, new PropertyChangedEventArgs(propertyName));
      }
   }
}

If an object of this type is bound directly to the DataContext (or ObjectDataProvider) the HasChanges property can be referenced directly in the xaml.

If the collection object is intantiated by a Model class the Model will need to implement INotifyPropertyChanged and expose its own HasChanges property with its value set by wiring up the ItemPropertyChanged event:

public MyObservableCollection Abc { get; set; }

public Model()
{
   Abc = new MyObservableCollection();
   Abc.ItemPropertyChanged += new PropertyChangedEventHandler(BusinessPropertyChanged);
}
public bool HasChanges
{
   get { return Abc.HasChanges; }
}
protected void BusinessPropertyChanged(object sender, PropertyChangedEventArgs e)
{
   OnPropertyChanged("HasChanges");
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
   if (PropertyChanged != null)
   {
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
   }
}