IDataExchangeCallback Callback
{
    get
    {
        return OperationContext.Current.GetCallbackChannel<IDataExchangeCallback>();
    }
}

protected void OnPropertyChanged(string propertyName)
{
    PropertyChangedEventHandler handler = PropertyChanged;

    if (handler != null)
    {
        Application.Current.Dispatcher.Invoke(() =>
            handler(this, new PropertyChangedEventArgs(propertyName)));
    }
}