按Shift+Enter键换行，按Enter键发送信息

 private void chatBoxSend_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Modifiers.CompareTo(Keys.Shift) == 0)    //按Shift+Enter换行
            {
                if (e.KeyCode == Keys.Enter)
                {
                    this.chatBoxSend.AppendText("\n");
                    SendKeys.Send("{BACKSPACE}");
                }
            }
            else if(e.KeyCode == Keys.Enter)    //按Enter键发送消息
            {
                btnSend_Click(sender, e);
                SendKeys.Send("{BACKSPACE}");
            }
        }