public class ViewModelBase : INotifyPropertyChanged
{
      #region INotifyPropertyChanged Members
 
      public event PropertyChangedEventHandler PropertyChanged;
 
      public void NotifyChanged(string propertyName)   
      {   
            if (string.IsNullOrEmpty(propertyName))   
                throw new ArgumentNullException("propertyName");   
 
            if (PropertyChanged != null)   
                PropertyChanged(this, new PropertyChangedEventArgspropertyName));   
       } 
 
      #endregion
}