private void cb1_KeyDown(object sender, KeyEventArgs e)
{
     cb1.IsDropDownOpen = true;

     if (e.Key.ToString() == "Return")
        this.btnMSDS_Click(sender, e);
     else if (cb1.Text.Length > 2)
        setItemsChemicalNames(this.dTable, cb1, Properties.Settings.Default.ConnectStringToExcel,
                                         " WHERE [F4] LIKE '" + cb1.Text + "%'");


}
void setItemsChemicalNames(DataTable dt, ComboBox cb, string pathToExcel, string filter = " WHERE [F4] IS NOT NULL")
{
     AddingIntoComboBox(cb, getExcelChemicalNameLIst(pathToExcel, filter));
}

private void AddingIntoComboBox(ComboBox cb, DataTable dt)
{
     cb.Items.Clear();
     foreach (DataRow dr in dt.Rows)
          cb.Items.Add(dr["F4"]);
}