if (e.ColumnIndex == dataGridView1.Columns["仓库名"].Index) {
    dataGridView1.BeginEdit(false);
    System.Windows.Forms.ComboBox c = dataGridView1.EditingControl as System.Windows.Forms.ComboBox;
    if (c != null) {
        c.DroppedDown = true;
    }
                
}

        private void dataGridView1_EditingControlShowing(object sender, DataGridViewEditingControlShowingEventArgs e) {
            if (dataGridView1.CurrentCell.OwningColumn is DataGridViewComboBoxColumn)
            {
                System.Windows.Forms.ComboBox combo = (System.Windows.Forms.ComboBox)e.Control;
                combo.KeyDown += new KeyEventHandler(combo_KeyDown);
            }
        }

        void combo_KeyDown(object sender, KeyEventArgs e) {
            System.Windows.Forms.ComboBox c = sender as System.Windows.Forms.ComboBox;
            if (c != null) {
                c.DroppedDown = true;
            }
        }