Hi,

You can do it by override the onCellClick on the DataGridView and then call
BeginEdit.
This will get the DateGridview to load the edit control for the selected cell.
Then you can get the current EditControl from the DataGridView control and
then you can set the DroppedDown property to true. Of cause only if the
column is a DataGridViewComboboxColumn so check for that that first.
Example:

private void dataGridView1_CellClick(object sender,
DataGridViewCellEventArgs e)
{
  if (dataGridView1.Columns[e.ColumnIndex].Name == "YourColumnName")
  {
     dataGridView1.SelectionMode = DataGridViewSelectionMode.CellSelect;
     dataGridView1.BeginEdit(true);
     DataGridViewComboBoxEditingControl comboboxEdit =
                   
(DataGridViewComboBoxEditingControl)dataGridView1.EditingControl;
     comboboxEdit.DroppedDown = true;
  }          
}

Regards
Rasmus