title: Set value of bound field to NULL using DataGridViewComboBoxColumn in DataGridView
body: <p>I have a DataGridViewComboBoxColumn in a DataGridView that's based on a lookup table.</p>
  
  <p>The ValueMember and DisplayMember fields are bound to string columns in the DataTable. All rows have a value for both fields - except for a special record where the value field is deliberately set to NULL.</p>
  
  <p>However, when I choose this record an empty string is used instead of DBNull.Value in the DataTable bound to the DataGridView. This is happening at data entry time, before the data is pushed to the database. </p>
  
  <p>I've checked out the DataGridViewComboBoxColumn and DataGridView classes - there appears to be no simple way to customise this behaviour. Does anybody know if this is possible?</p>