Update TextBox bound to DataGrid - binding

Question

I hope I can explain my problem properly.
I have a datagrid bound to an ObservableCollection object, and a TextBox bound to the selected item of my datagrid.
When I programmatically modify the SelectedItem property value (Name), my TextBox text value is not updated !
here is my design code:
<DataGrid Name="grid" HorizontalAlignment="Left" Margin="119,28,0,0" VerticalAlignment="Top" Height="237" Width="200" AutoGenerateColumns="False">
<DataGrid.Columns>
<DataGridTextColumn Header="nom" Binding="{Binding Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"/>
</DataGrid.Columns>
</DataGrid>
<TextBox Name="textbox" Text="{Binding ElementName=grid, Path=SelectedItem.Name, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" HorizontalAlignment="Left" Height="18" Margin="119,276,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="200"/>
<Button Content="Button" HorizontalAlignment="Left" Margin="392,54,0,0" VerticalAlignment="Top" Width="75" Click="Button_Click"/>
and here is my Code Behind:
ObservableCollection<Element> obs;
class Element
{
public string Name { get; set; }
public Element(string name) { Name = name; }
}
public MainWindow()
{
InitializeComponent();
obs = new ObservableCollection<Element>() { new Element("element2"), new Element("element2"), new Element("element3")};
grid.ItemsSource = obs;
}
private void Button_Click(object sender, RoutedEventArgs e)
{
var sel = grid.SelectedItem as Element;
sel.Name = "something";
grid.Items.Refresh(); //this updates the selected element to "something" but does nothing to the textbox
}

Answer 1

Problem solved.
I had to fire PropertyChanged event whenever a modification is made to the model.
here my new code:
class Element : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
string name;
public string Name
{
get
{
return name;
}
set
{
name = value;
PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Name"));
}
}
public Element(string name) { Name = name; }
}