private void textbox_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Down || e.KeyCode == Keys.Enter)
            {
                e.SuppressKeyPress = true;
                System.Windows.Forms.SendKeys.Send("{Tab}");
            }
            if (e.KeyCode == Keys.Up)
            {
                e.SuppressKeyPress = true;
                System.Windows.Forms.SendKeys.Send("+{Tab}");
            }
        }

------解决方案--------------------

还是有简便的方法的。
仅需在母窗体类上加入：
C# code
        protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
        {
            Keys key = (keyData & Keys.KeyCode);

            if (key == Keys.Down)
            {
                SendKeys.Send("{Tab}");
                return true;
            }
            return base.ProcessCmdKey(ref msg, keyData);
        }