Based on the fact that the TS mentioned populating the dropdown from the database, it's very unlikely that SelectedValue is actually a string.
More than likely, ValueMember is bound to the Id of the record in the database, DisplayMember is bound to some Description.
object val = ((ComboBox)sender).SelectedValue;
string txt = ((ComboBox)sender).SelectedText;
object item = ((ComboBox)sender).SelectedItem;
Depending on your needs, use any of these to retrieve/show further data.