Question

I have a DataGridViewComboBoxColumn with DropDownStyle = ComboBoxStyle.Simple

How can I force the dropdown to expand on a keystroke?

I have tried putting ((ComboBox)?).DroppedDown = true; in several places without solution

Answer 1

Following code snippet should work:

//If no row/cell selected, select an appropriate row/cell
dataGridView1.Rows[0].Selected = true;
dataGridView1.Rows[0].Cells[0].Selected = true;

//Start edit mode or otherwise EditingControl property will return null
dataGridView1.BeginEdit(true);

var comboBox = dataGridView1.EditingControl as DataGridViewComboBoxEditingControl;

if (comboBox != null)
{
   comboBox.DroppedDown = true;
}

Note that for this to work, there must be items in your combobox, either entered via binding or manually.