text box value won't updated using data bind

text box value won't updated using data bind

(new to WPF) in order to prevent long discussion in the comment section
I'm asking it again.
I have text box which bind to an object. When I'm changing the object
value the text box text value won't change. I read all most each
discussion exist here about this and didn't figure it out.
XAML:
<TextBox Height="36" HorizontalAlignment="Left" Margin="174,0,0,400"
Name="txtStudenName" VerticalAlignment="Bottom" Width="240"
IsEnabled="False" >
<TextBox.Text>
<Binding NotifyOnSourceUpdated="True"
Path="firstName"
UpdateSourceTrigger="PropertyChanged" >
</Binding>
</TextBox.Text>
codebehyond
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private student _selectedStudent;
public student selectedStudent // This is the object
{
get { return _selectedStudent; }
set
{
if (value != _selectedStudent)
{
_selectedStudent = value;
OnPropertyChanged("_selectedStudent");
}
}
}
public MainWindow()
{
InitializeComponent();
selectedStudent = clsLoadStudent();
this.DataContext = selectedStudent; // Here I use the object
as a datacontext.
}
private void dgStudents_SelectionChanged(object sender,
SelectionChangedEventArgs e)
{
selectedStudent = new student();
selectedStudent = (student)dgStudents.SelectedItem;
}
protected void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null) // if there is any subscribers
PropertyChanged(this, new
PropertyChangedEventArgs(propertyName));
}
}
I have been told to implement OnPropertyChanged... I did.. I think..