problem with binding a textBox

hi,
i'm trying to bind a simple textbox to a message string returned from object.
first i have a base class - 
public class BaseClass : INotifyPropertyChanged
{
 private string message
 public string Message
 { get {return message;}
    set
      { message = value;
        OnPropertyChaned("Message");
      }
 }
 
 public event PropertyChangedEventHandler propertyChanged;
 private void OnPropertyChanged(string propertyName)
 {
        if(propertyChanged!=null)
          propertyChanged(this, new PropertyChanedEventArgs(propertyName));
 }
}


Also, i have a sub-class - 
public Class SubClass : BaseClass
{
}


my viewModel class - 
public class ViewModel
{
  private BaseClass theClass;
  public ViewModel()
  {
    base=new BaseClass();
  }

  public BaseClass TheClass
  {
     get{ return theClass;}
     set{ theClass= value;}
  }

  private void someBtnClickMethod()
  {
     TheClass = new SubClass();
     theClass.Message="123";
  }
}


i'm binding the textBox with - 



the problem is that when someBtnClickMethod function call than i have to change the object and then binding not working for me, message is not updated in textBox.
what's the problem? how can i fix that?
thanks in advanced.