By default, toolbars and menus do not force validation for controls. Validation is a required part of validating and updating the data in a bound control. Once the form and all bound controls are validated, any current edits need to be committed. Finally, the table adapter needs to push its changes back to the database. To do this, put the following three lines in a click event handler on your form:

this.Validate();
this.customersBindingSource.EndEdit();
this.customersTableAdapter.Update(this.northwindDataSet.Customers);