Newbie: Simple Databinding in WinForms

Hi experts,

I have a small WinForms application that contains
- a DataGridView,
- a BindingNavigator,
- a BindingSource and
- a Textbox

The DataGridView shows commodities coming from a database and the BindingNavigator helps the user scrolling through the rows of commodities. DataGridView and BindingNavigator are connected to the same BindingSource. Everything works as expected.

However: I want the TextBox to show a certain value (i.e. the commodity's name) of the currently focused row in the DataGridView.

Thus I set a databinding in the Forms Load()-event like this:
            TextBox.DataBindings.Add("Text", commodityDataSource, "CommodityName");

When I start the application, the TextBox shows data from the first DataGridView row as expected.

But here is my problem: When I select another row in the DataGridView via Mouse or via Navigator the content of the TextBox is NOT updated.

What am I missing? Thanks for any help!
Ingmar