Silverlight binding does not work dynamically

Silverlight binding does not work dynamically
I am Silverlight newbie and I can't make the simple Silverlight binding sample work!
I need to make a view-model, that shows the number of documents in the list, while it is loading.
I made a base-class, that implements INotifyPropertyChanged:

public abstract class BaseViewModel : INotifyPropertyChanged {      protected BaseViewModel() {}       #region INotifyPropertyChanged Members      protected void OnPropertyChanged(string propertyName) {         PropertyChangedEventHandler handler = PropertyChanged;          if (handler != null)         {             handler(this, new PropertyChangedEventArgs(propertyName));         }     }      public event PropertyChangedEventHandler PropertyChanged;